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UNIVERSITY OF CINCINNATI

Date:______

I, ______, hereby submit this work as part of the requirements for the degree of: in:

It is entitled:

This work and its defense approved by:

Chair: ______

Developing Strategies for Use in the Pertechnetate Spectroelectrochemical Sensor:

+2 Studies of PVTAC-PVA and Metal(vbpy)3 Films

A dissertation submitted to the

Division of Research and Advanced Studies

of the University of Cincinnati

in partial fulfillment of the requirements

for the degree of

DOCTORATE OF PHILOSOPHY (Ph.D.)

in the Department of Chemistry

of the College of Arts and Sciences

2004

Jean Renee Paddock

B.S., University of Cincinnati, 2000

M.S., University of Cincinnati, 2003

Committee Chair: Carl J. Seliskar, Ph.D. Abstract

In years past, the Chemical Sensors research group at the University of Cincinnati

(U.C.) has been working on the problem of developing a spectroelectrochemical sensor

99 - for radioactive technetium-99 ( Tc) in the form of pertechnetate (TcO4 ). This project

has been of great interest to the U.S. Department of Energy (DOE) based on applicability

to its Hanford site (Pacific Northwest National Labs; Richland, WA). Serving partly as a

high-level nuclear waste storage facility, Hanford has seen leakage from aging waste

storage tanks and now deals with leakage plumes containing 99Tc which lie adjacent to

the Columbia River and associated water supply. Concern arises from the long half-life

99 5 - of Tc (2.15 x 10 years) and its fast migration though soil as TcO4 . The DOE is

interested in streamlining its soil and water quality laboratory-based tests (slow and

expensive) and in particular would like to see a remote, portable sensor able to detect

99Tc levels in situ or with point-of-interest continuous monitoring. Currently, such a

sensor does not exist.

Presented herein are several steps continuing the U.C. Chemical Sensors Group

- research towards a TcO4 chemical sensor. The sensor concept combines spectroscopy

- and electrochemistry to achieve selectivity (detecting TcO4 only) and sensitivity

(detecting low levels of occurrence) in one device. One of its major components is a

- chemically selective film which serves both as an initial level of TcO4 charge- or ligand-

based interaction and as its preconcentration device. This dissertation presents further

study on two films meant for use in the spectroelectrochemical sensor: poly(vinylbenzyltrimethylammonium chloride) blended with a poly(vinylalcohol) host

(PVTAC-PVA) and the ligand based film combining a metal with 4-vinyl-4´-methyl-

iii +2 2,2´-bipyridine (Metal(vbpy)3 ). The PVTAC-PVA film has been previously developed and presented herein are further, more specific studies of details affecting its performance in the sensor. The metal-ligand film is part of a promising direction involving ligand

- incorporation into a chemically selective film, thereby increasing TcO4 specificity and improving its spectroscopic and electrochemical properties. Additionally included are studies of new film substrates and the incorporation of sensor concepts into instructional laboratories for use in academics.

v Acknowledgments

To my family: I owe my love of learning and life to my mom and dad, brother,

and two sisters. Wherleys, you have shown me the value of unconditional love and given

me the courage to undertake any endeavor and work for success. You always make me

feel special, and to grow up with that kind of support and encouragement is a rare and

precious gift. An even greater rarity is finding that same spirit in my husband. I am

privileged to share my life with you Thomas. I could ask for nothing more than this

family.

To my friends: The journey of life is made colorful and adventuresome by the

company of friends and I have been blessed to know many. Thank you for making life

fun, even on days when fun was hard to find. You are each important to me and I thank

you for your constant support, interest, and willingness to share your experiences.

To my teachers at all levels: I hold your work as educators in the highest honor.

You are my guideposts and examples in my career and I strive to demonstrate excellence

you truly make me believe is within. My graduate advisory committee is my wise triumvirate, fostering my personal development, and helping me to achieve time and again. Carl, Bill, and Tom, I am thrilled to spend my career teaching and investing in others as I have learned from you.

I am a work in progress and my ability to complete the work represented in this

dissertation is attributed to those who have fostered my development as a student,

scientist, teacher, and human being.

vi Table of Contents

Approval Form i

Title Page ii

Abstract iii

Acknowledgments vi

Table of Contents 1

List of Figures and Tables 4

Chapter 1. Further Investigations on a Poly(vinylalcohol)-Polyelectrolyte

Chemically Selective Optical Film. 11

Introduction 11

Experimental 13

Results and Discussion 16

Conclusions 27

-4 Chapter 2. Diffusion of Fe(CN)6 in Chemically Selective Films. 28

Introduction 28

Experimental 29

Results and Discussion 30

Conclusions 40

-4 Chapter 3. Hybrid Optically Transparent Electrodes and Fe(CN)6 . 43

1 Introduction 43

Experimental 46

Results and Discussion 48

Conclusions 55

Chapter 4. 4-vinyl-4´-methyl-2,2´-bipyridine as a Ligand for Use in the

Spectroelectrochemical Sensor. 56

Introduction 56

Experimental 57

Results and Discussion 62

Conclusions 77

Chapter 5. An Instructional Laboratory for Making and Using a Material to

Sense Cu+2. 80

Introduction 80

Experimental 83

Results and Discussion 84

Conclusions 87

References 89

2 Appendix I. Determining Diffusion Coefficients of Analytes in a Thin Film

as Used in the Spectroelectrochemical Sensor:

A Step-by-Step Guide. 98

Appendix II. Instructional Laboratories as used in Freshman Honors

Chemistry, University of Cincinnati, Ohio. 103

Data Analysis 104

More Data Analysis 113

Boyle’s Law Revisited 120

The Spectrum of a Dye Molecule and the Lambert-Beer Law 131

Acid-Base Titrations 146

Spectrophotometric Determination of an Equilibrium Constant 155

Synthesis and Properties of Solid Solutions: Alums 162

Polymers, Gels, Ion-Exchange, and Stuff Like That 170

Crystals 185

3 List of Figures and Tables

Figure 1.1. Diagram showing the cross-section (not to scale) of a thin film in the

spectroelectrochemical sensor. 11

Figure 1.2. Electrochemical (A) and optical (B) response of a PVTAC-PVA blend

film exposed to 1.0 mM ferricyanide in 0.1 M KNO3 at 20 mV/s vs.

Ag/AgCl reference. Old PVA denotes a film made with more than one

month old PVA. Fresh PVA solution (less than 1 week old) is denoted as

new PVA. 16

Figure 1.3. Electrochemical (A) and optical (B) response of a PVTAC-PVA blend

film exposed to 1.0 mM ferricyanide in 0.1 M KNO3 at 20 mV/s vs.

Ag/AgCl reference. Unstirred blend was mixed with a pipet tip only.

Stirred blend utilized a stir bar and approximately 15-20 seconds of

mechanized stirring. 17

Figure 1.4. Refractive index (n) and extinction coefficient (k) of PVA, PVTAC, and

blend film, PVTAC-PVA, all coated on Schott SF11 glass. Films consist

of either pure PVA, pure PVTAC, or a PVTAC-PVA blend (1:4.3 mass

ratio). Values are a result of spectroscopic ellipsometric data analysis.

Figure 1.5. Electrochemical (A) and optical response (B) of PVTAC-PVA blend films

composed of varying %PVTAC and exposed to 1.0 mM ferricyanide in

0.1 M KNO3 at 20 mV/s vs. Ag/AgCl reference. Cyclic voltammetry

scans began in the negative direction for this figure and optical data

4 presented shows initially absorbent ferricyanide instead of the typical

scenario wherein non-absorbent ferrocyanide is present at the beginning of

a positive direction electrochemical scan. 20

Figure 1.6. PVTAC-PVA film (Schott SF11 base glass) thickness changes induced by

exposure to 0.1 M KNO3. Refractive index values, n, at 450 nm of both

the dry and equilibrated film are shown. 22

Figure 1.7. PVTAC-PVA film (Schott SF11 base glass) experimental ellipsometric

parameter Ψ changes in time observed during exchange 0.1 M KNO3 with

identical solution containing 1 mM ferricyanide. Arrows indicate

approximate time when ferricyanide injections occur. Film thickness and

refractive index (at 450 nm) prior to injection and after equilibration with

ferricyanide solution are shown. 24

Figure 1.8. PVTAC-PVA film on ITO coated 1737F glass exposed to 1 x 10-6 M

fluorescein in 0.1 M KNO3 with fluorescence monitored at 520 nm in

time. The film was equilibrated in 0.1M KNO3 for 18 hours prior to

experiment. Solution exchange sequence is as follows: (a) 8 min,

injection of fluorescein solution, (b) 37 min, injection of 1 M KNO3

solution. 26

Table 1.1. Thickness, t, refractive index, n at 450 nm, and PVTAC:PVA monomer

ratio for PVTAC-PVA blend air dry films with varied proportions of

PVTAC by mass. 19

Figure 2.1. Saturation of a PVTAC-PVA film coated on ITO and exposed to 0.1 mM

-3 Fe(CN)6 in 0.1 M KNO3. (A) Shows monitoring of the event with cyclic

5 voltammograms regularly recorded (800 to -300 mV, scan rate 20 mV/s

vs. Ag/AgCl reference). (B) Records cathodic peak current and the

extrapolation of a saturation time. 31

Figure 2.2. Chronoamperogram for PVTAC-PVA film on ITO preconcentrated with

-3 0.1 mM Fe(CN)6 . Pulse: 800 to -300 mV, 50 msec width vs. Ag/AgCl

reference. Raw data shown in (A). Cottrell plot necessary for calculation

shown in (B) with Cottrell slope of region (2) only shown as a subset.

Figure 2.3. Chronoamperogram for PVTAC-PVA film on ITO preconcentrated with

-3 Fe(CN)6 (Figure 2.2). Includes effects due to 0.1 M KNO3 in H2O as raw data and the subtracted difference between the two. Pulse: 800 to -

300 mV, 50 msec width, vs. Ag/AgCl reference. Raw data shown in (A).

Cottrell plot necessary for calculation shown in (B). 35

Figure 2.4. Randles-Sevcik plot for a PVTAC-PVA film loaded with 0.1 mM

-3 Fe(CN)6 . Data points polynomially fitted (B) shown are derived from

cyclic voltammograms at 0.001, 0.005, 0.025, 0.050, 0.10, 0.25, 0.50,

0.75, 1.0, 5.0, and 10.0 V/s (A), all vs. Ag/AgCl reference. 37

Figure 2.5. Chronoamperogram for PVTAC-PVA film on ITO preconcentrated with

-3 Fe(CN)6 . Pulse: 800 to -300 mV, 32 sec width vs. Ag/AgCl reference.

Raw data shown in (A). Cottrell plot necessary for calculation shown in

(B). 38

Figure 2.6. Chronoamperogram for PVTAC-PVA film on ITO preconcentrated with

-3 Fe(CN)6 (Figure 2.5). Includes effects due to 0.1 M KNO3 in H2O as raw data and the subtracted difference between the two. Pulse: 800 to -

6 300 mV, 32 sec width vs. Ag/AgCl reference. Raw data shown in (A).

Cottrell plot necessary for calculation shown in (B). 39

Figure 3.1. Accessible potential windows in 0.1M KNO3 for each of four hybrid

electrodes: (A) Au/ITO, (B) C/ITO, (C) Pt/ITO, and (D) Pd/ITO.

-3 Figure 3.2. Cyclic voltammograms for Fe(CN)6 on ITO hybrid and bare ITO

electrodes (1.0 mM in 0.1M KNO3, vs. Ag/AgCl, 20 mV/s): (A) Au/ITO, (B) C/ITO, (C) Pt/ITO, and (D) Pd/ITO. 51

Figure 3.3. Sweep rate study showing cathodic and anodic peak currents for

potassium ferricyanide coupled with each of three hybrid electrodes: (A)

Au/ITO, (B) C/ITO, and (C) Pt/ITO. 54

Table 3.1. Peak current potential difference and cathodic/anodic peak current ratios

for potassium ferricyanide (K3Fe(CN)6) coupled with all working

electrodes studied. 52

1 +2 Figure 4.1. H NMR spectra of Fe(vbpy)3(PF6)2 and Zn(vbpy)3(PF6)2. General M -

ligand structure is shown with assigned peaks. Solvents are water,

acetonitrile, and tetramethylsilane reference. 60

Figure 4.2. Bird’s eye view of physical set-up used for reflection based

measurements. (A) and (C) represent quartz core optical fibers used with

collimators. (B) is a 6-fiber bundle fluorescence detection cable. Fibers

are aligned such that the light source (fiber A) is directed onto the Pt core

of the disk electrode and the specular reflection is collected by fiber C.

7 +2 +2 Figure 4.3. Absorbance spectra for 0.01mM vbpy, Fe(vbpy)3 , Zn(vbpy)3 , and

FeCl2, all solutions in 0.1M tBAPF6/CH3CN. 63

+2 Figure 4.4. Fluorescence spectra of 0.005mM Zn(vbpy)3 in 0.1M tBAPF6/CH3CN.

+2 Figure 4.5. Electropolymerization of 3mM Fe(vbpy)3 in 0.1M tBAPF6/CH3CN vs.

quasi Ag/AgCl reference at 100 mV/s (A), and investigation of the

+2 resultant Fe(vbpy)3 film on a Pt disk electrode in supporting electrolyte

only (B). Both cyclic voltammograms include a scan using a bare Pt disk

electrode in supporting electrolyte only. 66

Figure 4.6. BAS Pt disk electrode as seen under 10X magnification before (A) and

+2 after (B) Fe(vbpy)3 film polymerization. 66

+2 Figure 4.7. Electropolymerization of 3mM Fe(vbpy)3 in 0.1M tBAPF6/CH3CN vs.

quasi Ag/AgCl reference at 100 mV/s (A), and investigation of the

+2 resultant Fe(vbpy)3 film on an ITO slide working electrode in supporting

electrolyte only (B). Both cyclic voltammograms include a scan of a bare

ITO slide in supporting electrolyte only. 67

Figure 4.8. Optically transparent ITO electrode as seen under 10X magnification

+2 before (A) and after (B) Fe(vbpy)3 film polymerization. 67

+2 Figure 4.9. Electropolymerization of 3mM Fe(vbpy)3 in 0.1M tBAPF6/CH3CN vs.

quasi Ag/AgCl reference at 100 mV/s (A), and investigation of the

+2 resultant Fe(vbpy)3 film on a Pt-ITO hybrid electrode in supporting

electrolyte only (B). Microscopy of the resultant film at 10x

8 magnification is also shown (C). Both cyclic voltammograms include a

scan of a bare Pt-ITO hybrid electrode in supporting electrolyte only.

+2 Figure 4.10. Electropolymerization of 3mM Fe(vbpy)3 in 0.1M tBAPF6/CH3CN vs.

quasi Ag/AgCl reference at 100 mV/s (A), and investigation of the

+2 resultant Fe(vbpy)3 film on a Au-ITO hybrid electrode in supporting

electrolyte only (B). Both cyclic voltammograms include a scan of a bare

Au-ITO hybrid electrode in supporting electrolyte only. 70

Figure 4.11. Optically transparent Pt-ITO hybrid electrode (A) and Au-ITO electrode

+2 (B) as seen under 10X magnification and coated with an Fe(vbpy)3 film.

+2 Figure 4.12. Electropolymerization of 3mM Zn(vbpy)3 in 0.1M tBAPF6/CH3CN vs.

quasi Ag/AgCl reference at 100 mV/s (A), and investigation of the

+2 resultant Zn(vbpy)3 film on a Pt disk electrode in supporting electrolyte

only (B). Both cyclic voltammograms include a scan of a bare Pt disk

electrode in supporting electrolyte only. 72

Figure 4.13. Exchange of Zn+2 for Fe+2 in vbpy based films detailed. (A) Shows 2

+2 examples from 5 cycles during exposure of Zn(vbpy)3 films to 10 mM

FeCl2 in 0.1 M tBAPF6/CH3CN vs. quasi Ag/AgCl reference at 100 mV/s.

+2 (B) Shows the newly formed Fe(vbpy)3 film alongside bare Pt disk and

+2 previous Zn(vbpy)3 film. Both cyclic voltammograms include a scan of

a bare Pt disk electrode in supporting electrolyte only. 74

9 Figure 4.14. BAS Pt disk electrode as seen under 10X magnification detailing the

+2 +2 +2 formation of a Zn(vbpy)3 film and exchange of Zn with Fe to form a

+2 film of Fe(vbpy)3 . 74

Figure 4.15. Transmission (A) and Absorbance (B) data collected during the exchange

+2 +2 +2 of Zn for Fe thereby forming Fe(vbpy)3 films on a Pt disk electrode.

Figure 4.16. Fluorescence emission data collected during the exchange of Zn+2 for Fe+2

+2 thereby forming Fe(vbpy)3 films on a Pt disk electrode. Integration time

was 100 msec and laser power was ≤ 0.1 mW. 77

Figure 5.1. Crosslinking of PVA with glutaraldehyde. Network entrapped PAA

(protonated form) is shown at bottom. 82

-5 Figure 5.2. Absorbance spectra of 8x10 M PAN, 0.05M CuSO4, and resultant PAN-

Cu complex. PAN-Metal+2 complex is shown at right. 85

Figure 5.3. Absorbance spectra of PVA-PAA/PAN network sections soaked for 1

hour in CuSO4 solution at a concentration of (a) 0.025 mM, (b) 0.05 mM,

Figure 5.4 Calibration plot for the PAN-Cu complex at λmax = 555 nm (from spectra

shown in Figure 5.3). 87

10 Chapter 1.

Further Investigations on a Poly(vinylalcohol)-Polyelectrolyte

Chemically-Selective Optical Film

Introduction

A spectroelectrochemical sensor utilizing three modes of selectivity has been

developed by our group.1-10 The sensor in its most common form consists of a multilayer

assembly of transparent thin films anchored on a slab of optical glass as shown in Figure

1.1.

aqueous phase

chem-selective film

ITO

optical glass

Figure 1.1. Diagram showing the cross-section (not to scale) of a thin film in the

spectroelectrochemical sensor.

Detection is by multiple total internal reflection at a wavelength of choice as the potential of the electrode is modulated. Of the sensor films, the one that consists of a porous optical material (chem-selective film) plays a crucial role in preconcentrating the target analyte at the surface of a transparent electrode (usually a thin film of indium tin oxide).

11 To date we have developed two different classes of such porous optical materials. One type11 consists of sol-gel processed silica composites containing polyelectrolytes and these films have served as benchmarks for development of the sensor. The other type12 is based on cross-linked poly(vinylalcohol) in which is entrapped a long-chain polyelectrolyte to form an optically clear hydrophilic network. These polymeric materials may be covalently attached to an oxide surface providing ruggedness not available with our sol-gel processed films. In the course of designing spectroelectrochemical sensors for radioactive waste materials, we have discovered that one polymeric film material, namely, that consisting of poly(vinylalcohol) entrapped poly(vinylbenzyltrimethylammonium chloride), designated herein as PVTAC-PVA, shows very promising characteristics with respect to preconcentrating pertechnetate

99 − 13 ( TcO4 ) which is a substance of concern at nuclear waste sites nationwide. It, therefore, became important to more carefully study the detailed physical and optical properties of this new film material for possible use in sensors at such sites. Because of the current direction of our work, it also became necessary to investigate both the chemical reversibility and the compatibility with fluorescence detection of the film.

A formulation for PVTAC-PVA has been reported by us previously12 along with its general behavior as a chemically selective optical material. This composite material acts as an ion exchanger absorbing negatively charged species that are small enough to penetrate the hydrophilic network established by the cross-linked poly(vinylalcohol) host material. PVTAC, a linear polymer, is entrapped within the PVA host material to an extent that we have been unable to measure any leaching of it over the useful time of a sensor with that film. This paper presents a more detailed characterization of the optical

12 properties of PVTAC-PVA, along with an in situ investigation of its dynamic physical

behavior under aqueous conditions that are similar to those in an actual sensing

application. Additionally, chemical reversibility of the film is investigated in a system utilizing fluorescence as a means of detection. In turn, these studies provide a more complete understanding of PVTAC-PVA as a chemically selective optical component of the spectroelectrochemical sensor.

Experimental

Chemicals and materials. Poly(vinylalcohol) (PVA) (MW 85,000-146,000, 98-

99% hydrolyzed) and 3-aminopropyltriethoxysilane (APTS) were purchased from

Aldrich. Glutaraldehyde (reagent grade, 50% w/w), hydrochloric acid (ACS reagent), buffers (pH 4, 7, and 10), potassium nitrate, potassium ferricyanide, sodium acetate, and sodium hydroxide were purchased from Fisher. Poly(vinylbenzyltrimethylammonium chloride) (PVTAC) (30 wt% in water, MW 400,000) was purchased from Scientific

Polymer Products. Fluorescein (sodium salt) was purchased from ICN Biomedicals.

Stock solutions for polymer blends were prepared by dilution of commercial solutions or by dissolution of solid polymer in deionized water.

Indium tin oxide (ITO)-coated glass (11-50 Ω/sq, ~135 nm thick ITO layer over

Corning 1737F glass) was purchased from Thin Film Devices and diced into 10 mm x 45 mm pieces (slides). SF11 glass (nd = 1.78) purchased from Schott was cut and polished

as previously described14 and used as a substrate for all detailed ellipsometric film

investigations.

13 Thin film preparation. Preparation of PVTAC-PVA films was done over three

days. On day one, ITO-coated slides were washed thoroughly (soap and water wash,

deionized water rinse, ethanol rinse, deionized water rinse) and placed upright into a

beaker of 2 M NaOH, covered, and left overnight. On day two, these slides were rinsed

with deionized water and placed in a beaker of 5% APTS in sodium acetate buffer (pH

5.5), which was covered and heated overnight to a solution temperature between 85 and

95oC. On day three, slides were rinsed with deionized water and dried by a 30 second

spin at 5000 rpm (Photo Resist Spinner, Model #1-PM101DT-R485 from Headway

Research, Inc.). 100 µL of polymer blend was then evenly spread onto the ITO-coated

surface of a 10 mm x 45 mm glass substrate and spun at 5000 rpm for 30 seconds. Thin films on SF11 glass were made by a procedure identical to that for ITO-coated slides described above.

One polymer blend batch consisted of 2.50 mL 10% (w/w) PVA, 0.50 mL 15%

(w/w) PVTAC, 1.50 mL 5% (w/w) glutaraldehyde, and 0.20 mL 0.5M HCl in deionized water.12 This blend was carefully mixed with a stir bar (avoiding bubbles) until

homogeneous (5-10 seconds) prior to spin coating. Approximately 3-4 slides could be made per batch of polymer blend. To vary the percentage of PVTAC present in the film,

0.50 mL of a solution of the desired percentage of PVTAC was substituted for the standard 15% solution in the above recipe.

Film thickness, refractive index, and dynamic in situ measurements were obtained using standard ellipsometric methods on a J.A. Woollam Variable Angle Spectroscopic

Ellipsometer.15 Spectroscopic ellipsometry measures the polarization and intensity of a

beam of light of wavelength λ reflected from a surface, and from measured quantities

14 (intensity, polarization) the ellipsometric parameters Ψ and ∆ are calculated. Ψ and ∆ are

ratios of the intensity and phase, respectively, of the reflected components in the plane

and perpendicular to the plane of incidence.15 Film thickness and refractive index values

for films on ITO-glass (Corning 1737F glass) were determined using an optical layer

model that consisted of an isotropic film layer, a graded layer of ITO (133 nm), and a

base glass (1737F) layer (1 mm thickness). Film thicknesses and refractive index values

on Schott SF11 glass were determined using an optical layer model that consisted of a

solution layer of infinite thickness, an isotropic film layer, a SF11 layer, and a surface

roughness layer (5 nm). These ellipsometric measurements provide a much greater

degree of reliability compared to those values reported earlier by this group.12 Dynamic measurements of film properties were done using a method recently developed by us.14, 16

Spectroelectrochemical measurements were performed using Specwonder, an in-house developed system for making simultaneous electrochemical and spectroscopic measurements.1-10

Fluorescence measurements. Fluorescence measurements were performed on

an instrument assembled using multiple internal reflection optics as described

previously.17 Film preparation prior to measurements involved hydration of the PVTAC-

PVA film in 0.1M KNO3 for 18 hours. Fluorescein was used at a concentration of 1 x

-6 10 M in 0.1 M KNO3. A 441.6 nm single mode HeCd laser (Kimmon Electric) served

as excitation source which was attenuated to 0.1 mW for fluorescence excitation. A

syringe pump (Sage Instruments) controlled solution flow into the fluorescence cell at

0.23 mL/min with fluorescence photon counts collected every 1000 ms with 50 ms integration time.

15 Results and Discussion

Film optical properties. A general formulation for PVTAC-PVA was previously

developed, balancing needs for good adhesion to the substrate surface, optical

transparency, ion-exchange capacity, ruggedness, pore network size, film thickness, and

ease of working with materials.12 A typical film precursor solution is blended from

aqueous stock solutions of 15% PVTAC and 10% PVA in a 1:4.3 dry mass ratio. Such a

blend on spin coating produces a film with an average thickness of 709 nm, although it

should be noted that standard deviation of this film thickness approaches 20%. The film

is coated using a curing reaction and film thickness will depend explicitly on the timing

of spin coating, as well as humidity, homogeneity of the mixture, and individual

technique. It has been found that PVA solutions less than one month old and blend

solutions which have been stirred produce films of the highest quality with optimal

performance. Figures 1.2 and 1.3 illustrate the differences seen in a typical

spectroelectrochemical measurement using more than one month old PVA versus a fresh

solution and a homogeneous stirred blend mixture versus no stirring, respectively.

300 A 0.14 B

200 bare ITO 0.12 ITO + old PVA ITO + new PVA 100 0.10 A µ 0 0.08

-100

Current, 0.06 Absorbance, AU

-200 bare ITO 0.04 ITO + old PVA -300 ITO + new PVA 0.02

-400 0.00 0.8 0.6 0.4 0.2 0.0 -0.2 -0.4 0 20406080 Potential, V Time, s

Figure 1.2. Electrochemical (A) and optical (B) response of a PVTAC-PVA blend

film exposed to 1.0 mM ferricyanide in 0.1 M KNO3 at 20 mV/s vs.

16 Ag/AgCl reference. Old PVA denotes a film made with more than one

month old PVA. Fresh PVA solution (less than 1 week old) is denoted as

new PVA.

300 A 0.35 B 200 0.30

100

A 0.25 µ 0 0.20

Current, -100 0.15 Absorbance, AU Absorbance, -200 bare ITO 0.10 bare ITO ITO + unstirred blend ITO + unstirred blend -300 ITO + stirred blend 0.05 ITO + stirred blend

-400 0.00 0.8 0.6 0.4 0.2 0.0 -0.2 -0.4 0 20 40 60 80 100 Potential, V Time, s

Figure 1.3. Electrochemical (A) and optical (B) response of a PVTAC-PVA blend

film exposed to 1.0 mM ferricyanide in 0.1 M KNO3 at 20 mV/s vs.

Ag/AgCl reference. Unstirred blend was mixed with a pipet tip only.

Stirred blend utilized a stir bar and approximately 15-20 seconds of

mechanized stirring.

Overall, both figures show less preconcentration of analyte into the film with lower peak

currents in the cyclic voltammogram (panel A) and decreased absorbance (panel B).

Both responses are consistent with a film that is less homogeneous in nature. Following

this study, only fresh PVA was used in a stirred blend in order to maximize experimental film performance.

Non-uniformity of the cured film with optimized conditions is typically less than

10% as judged by modeling ellipsometric data. The useful time of a typical film in supporting electrolyte extends up to 36 hours or more with little change in film stability.

17 At this point, mottling and cracking are visible on the edges of the film, but such

imperfections do not penetrate beyond the edges. Past 48 hours, the typical overall

condition of the film begins to deteriorate until ultimately, delamination occurs.

The general optical properties of these films are by and large what would be

predicted by effective medium theory,15 namely, the optical properties of the polymeric composite in cured film form are intermediate between the properties of the neat component films. For example, the complex refractive index, ñ = n − ik, with constants n

and k shown in Figure 1.4 for air-dry film materials. (The reader should note that

absolute values of these constants may vary in air depending on the ambient percent

relative humidity.)

1.64 0.0008

1.62

0.0006 k Coefficient, Extinction 1.60

1.58 0.0004

1.56 PVTAC, n

1.54 PVTAC, k 0.0002 Index of Refraction, n Refraction, of Index PVA and Blend, n 1.52 PVA and Blend, k 0.0000 1.50

200 400 600 800 1000 1200

Wavelength, nm

Figure 1.4. Refractive index (n) and extinction coefficient (k) of PVA, PVTAC, and

blend film, PVTAC-PVA, all coated on Schott SF11 glass. Films consist

18 of either pure PVA, pure PVTAC, or a PVTAC-PVA blend (1:4.3 mass

ratio). Values are a result of spectroscopic ellipsometric data analysis.

For host material PVA, n shows the expected normal (Cauchy) dispersion of a non-

absorbing (k = 0) aliphatic material over the wavelength region 300 – 1100 nm. Thin

films of PVTAC, on the other hand, have higher values of both n and k over this region

but still display the normal dispersion of both constants. The increased values

undoubtedly are due to the proximity of the strong absorption in the near-UV of the

aromatic groups of the polymer and the associated linkage of n and k by the Kramers-

Kronig relation.15 Films made with varied percentages of PVTAC in an increasing

monomer ratio with PVA show expected slight increases in n relative to host PVA as

PVTAC content increases as shown in Table 1.1, below.

% PVTAC t, nm n, 450nm monomer ratio, PVTAC:PVA

0% 383 1.5251 0

5% 559 1.5299 0.016:1

10% 590 1.5331 0.032:1

15% 709 1.5397 0.048:1

20% 943 1.5414 0.065:1

25% 1064 1.5401 0.081:1

30% 1151 1.5434 0.098:1

Table 1.1. Thickness, t, refractive index, n at 450 nm, and PVTAC:PVA monomer

ratio for PVTAC-PVA blend air dry films with varied proportions of

PVTAC by mass.

19 As a practical consequence of increasing the proportion of PVTAC in the blend

solution prior to spin coating, the coating solution becomes more viscous and as a result

thicker films are produced at the same spin rate. However, because of the overall low

proportion by monomeric unit of PVTAC in host PVA, the optical constants of the blends

are only slightly elevated above those of the host material PVA.

It is to be noted that our first published film thicknesses and refractive index

values12 were determined by optical fringe18 and Abbe refractometry methods, respectively. However, neither of these methods is as accurate as the spectroscopic ellipsometric method and we report significantly better values for these constants here.

These better values are required for designing optical sensors incorporating this film in either an evanescent wave or waveguide mode of operation.

Films made with 0 to 30% (w/w) PVTAC and subsequent exposure to supporting

electrolyte show an incremental decrease in physical stability proportional to

increasing percentage of PVTAC. Within this same variation the exchange capacity of

the material increases directly with increasing PVTAC content (Figure 1.5).

800 A B 0% PVTAC 600 0.5 5% PVTAC 10% PVTAC 400 15% PVTAC 0.4 20% PVTAC bare ITO 200 25% PVTAC A

µ 30% PVTAC 0.3 0

Current, Current, 0% PVTAC -200

5% PVTAC Absorbance, AU 0.2 10% PVTAC bare ITO -400 15% PVTAC 20% PVTAC 0.1 -600 25% PVTAC 30% PVTAC -800 0.0 0.8 0.6 0.4 0.2 0.0 -0.2 -0.4 0 20406080100 Potential, V Time, s

Figure 1.5. Electrochemical (A) and optical response (B) of PVTAC-PVA blend films

composed of varying %PVTAC and exposed to 1.0 mM ferricyanide in

20 0.1 M KNO3 at 20 mV/s vs. Ag/AgCl reference. Cyclic voltammetry

scans began in the negative direction for this figure and optical data

presented shows initially absorbent ferricyanide instead of the typical

scenario wherein non-absorbent ferrocyanide is present at the beginning of

a positive direction electrochemical scan.

An acceptable balance is struck between these two offsetting trends at about 15%

PVTAC. With a greater amount of PVTAC present, the PVA network might be somewhat more stretched on hydration (vide infra), perhaps to a point where swelling of the film produces an expansion not able to be compensated for by the PVA host network.19

Film dynamics. The spectroelectrochemical sensor under development in our group

typically operates in an aqueous environment. Thus, understanding film behavior

before, during, and following hydration, or changes in hydration, is critical not only to the

evaluation of sensor performance but also in the design of new and better film materials.

In recent reports we have described a new experimental technique to study, in situ, film dynamics using spectroscopic ellipsometry, even to the point of being able to deduce analyte diffusion profiles within films.16 In order to begin to understand the physical

behavior of PVTAC-PVA exposed to a variety of aqueous conditions, we made films on a high quality optical glass (Schott SF11). We then studied film thickness and refractive index (at 450 nm) under different and changing aqueous environments.

21 When a dry film was exposed to 0.1 M KNO3 (used as supporting electrolyte in the sensor), and monitored ellipsometrically at 450 nm, significant changes in film thickness were measured as shown in Figure 1.6.

1260

1250

Soaked film 1240 n=1.454

1230

1220 Film's thickness, nm

795 Dry film 790 n=1.544 785 02468101214161820 Time, h

Figure 1.6. PVTAC-PVA film (Schott SF11 base glass) thickness changes induced by

exposure to 0.1 M KNO3. Refractive index values, n, at 450 nm of both

the dry and equilibrated film are shown.

After an air-dry film comes into contact with solution, rapid swelling occurs. Because of the limited time resolution of our ellipsometric measurements, this initial expansion cannot be time resolved adequately at early times (less than 30 minutes). In the first hour of hydration, the film reaches 99% of its largest expansion that occurs at about 3-4 hours of exposure. After full expansion to 1.6 times that of its original thickness, the film undergoes an approximately 1% slow contraction approaching a steady state value about

22 15-20 hours after initial exposure to solution. Incorporation of water into the film results

in an overall lowering of the film’s n value as would generally be predicted by effective

medium theory. That is, the solution equilibrated film refractive index (n = 1.454) falls between the refractive index of the dry film (n = 1.544) and water (n = 1.342). However, it is to be noted that these refractive index values are close to those of many common

optical glass and silica materials and, thus, in designing optical sensors incorporating this

film reasonably exact values of the film indices under use must be known.

It is interesting to examine the physical changes in the film on incorporation of a

multivalent anion. In an experiment to gauge these changes, a film equilibrated in 0.1 M

KNO3, was exposed to a sequence of two injections of a 0.1 M KNO3 solution containing

3− 1 mM ferricyanide (Fe(CN)6 ). Ellipsometric measurements at 450 nm were logged

before, during and after these injections with Figure 1.7 showing how the film

responded. After injection #1, a rapid, small contraction in film thickness (displayed as

3− rapid changes in ellipsometry parameter,Ψ) takes place as Fe(CN)6 partitions into the

− film replacing NO3 ions. The contraction of the film from its thickness of 1092 nm on

incorporation of ferricyanide occurs quite quickly (< 1 minute) and reaches a steady state.

The second bolus of ferricyanide produces a slighter response with about the same response time and the film re-stabilizes at a more slightly contracted thickness of 1053 nm (an overall change of 39 nm). A very small increase in refractive index, n, at 450 nm

is observed overall following the two injections (1.446 to 1.455), even with the presence

of 450 nm light absorbing ferricyanide in the film.

23 26.8

1053 nm 26.6 n=1.455 injection #2 26.4

, degrees 26.2 Ψ injection #1

26.0 1092 nm, n=1.446

-101234 Time, min

Figure 1.7. PVTAC-PVA film (Schott SF11 base glass) experimental ellipsometric

parameter Ψ changes in time observed during exchange 0.1 M KNO3 with

identical solution containing 1 mM ferricyanide. Arrows indicate

approximate time when ferricyanide injections occur. Film thickness and

refractive index (at 450 nm) prior to injection and after equilibration with

ferricyanide solution are shown.

Contraction of film thickness following incorporation of a multivalent ion has also been seen by us with Nafion (a highly cross-linked film). In both cases, experimental data shows no measurable amount of leaching either of Nafion or PVTAC from each respective film. The small contraction is probably a result of electrostatic cross-linking

− by incorporation of the multivalent ferricyanide in place of a univalent anion (NO3 ). It

24 seems less likely that the effect could be driven by osmotic contraction of PVA since the

ionic strength change on injection of ferricyanide is insignificant.19

Films used by this group as components of a chemical sensor are typically pre-

equilibrated in aqueous solution for up to 18 hours prior to use to avoid major effects

seen early on during hydration of the dry material. Nonetheless, in the post 4-hour time

period following initial hydration of PVTAC-PVA, a relatively small percentage change

(approximately 4%) is seen in film thickness. A suitable working curve can be made and

quantitation achieved after this 4-hour period. We note that such issues of film

expansion and contraction are not unique to this material and are probably common to

nearly all sensor film materials.

Fluorescence detection and film regeneration. Used in a sensor, a chemically

selective film is typically chosen for its high preconcentration characteristic and, thus, its

ability to lead to low limits of detection. But this often comes with an attendant cost, namely, that the film is difficult to reverse chemically for continued use. For this reason it is important to evaluate a new sensor material for reversibility. Performance of

-3 PVTAC-PVA films in the spectroelectrochemical sensor with ferricyanide (Fe(CN)6 ) has been reported previously.12 With recent work in our group, fluorescence is being investigated as a means of detection in the sensor system with the ability to lower limits of detection from the current absorbance based system.17 We chose the fluorescent anion

fluorescein to investigate the reversibility of films. A PVTAC-PVA film (t = 790 nm

dry) was exposed to 0.1M KNO3 for 18 hours prior to exposure to alternating solutions of

−6 fluorescein (1 x 10 M in 0.1 M KNO3), 1.0 M KNO3, and 0.1 M KNO3. The results are

shown in Figure 1.8.

b 20000

15000

10000

Photon Counts

5000 a

0 0 1020304050 Time, min

Figure 1.8. PVTAC-PVA film on ITO coated 1737F glass exposed to 1 x 10-6 M

fluorescein in 0.1 M KNO3 with fluorescence monitored at 520 nm in

time. The film was equilibrated in 0.1M KNO3 for 18 hours prior to

experiment. Solution exchange sequence is as follows: (a) 8 min,

injection of fluorescein solution, (b) 37 min, injection of 1 M KNO3

solution.

Incorporation of fluorescein into the film and the reaching of steady state took place in

about 6 min. With an injection of 1.0 M KNO3 (at 37 min into the measurements), nearly

complete reversal of fluorescein incorporation occurs. A subsequent re-equilibration (not

shown) of the film with 0.1 M KNO3 shows a small spike in fluorescence attributed to a refractive index change based on a change in ionic strength from 1 M to 0.1 M salt; however, the film returns completely to its secondary baseline. The small residual concentration of fluorescein seems much more tightly bound and not readily reversed by

26 additional salt solution exposure. It is interesting that regeneration of the film is fast and

a return to within 1% of the original baseline is promising for the development of a multi-

use reversible chemically selective film in the spectroelectrochemical sensor.

Conclusions

The PVTAC-PVA material forms high optical quality anion exchange films that

have been characterized by measuring optical constants as a function of wavelength over

the region 300 – 1100 nm. Real-time ellipsometry studies of films undergoing hydration

in dilute KNO3 show a large (about 160%) film expansion relative to the air dry state.

Incorporation of a multivalent anion into the hydrated film shows that the film contracts slightly probably due to electrostatic cross-linking. This behavior is not unique to

PVTAC-PVA but these physical property changes have rarely been reported for similar

sensor materials. When films containing a typical anion, for example, fluorescein, are

exposed to a more highly concentrated salt solution, the incorporation of the anion can be

almost completely reversed. These physical and optical properties make the PVTAC-

PVA material a promising optical material for incorporation into chemical sensors for

anions.

27 Chapter 2.

-4 Diffusion of Fe(CN)6 in Chemically Selective Films.

Introduction

As discussed in the previous chapter, the quest in understanding

spectroelectrochemical sensor behavior relies heavily upon understanding the behavior of

films used as preconcentration and selectivity devices therein. Thin films used in this

group are dynamic systems, constantly changing and adjusting to solution conditions.16,20-

23 One critical phase of sensor performance takes place when the film is introduced to a

solution containing the charged particles it will incorporate and preconcentrate.

Introduction of ions into the film is not unlike a crowd rushing in to obtain seating in an

empty auditorium and then filling in remaining spots. This movement of ions into and

within the film is governed largely by diffusion. By understanding and measuring the

rate of diffusion (diffusion coefficient) of a particular ion within a film, one begins to

more thoroughly understand the behavior of that film and how it might affect sensor

performance.

The diffusion coefficient represents a speed, or rate of movement for a specific

situation. The most straightforward and simplistic is that of a particle in aqueous

solution, and measurements of this nature have been made and are well-understood,

enough so that advanced high school students could follow a laboratory procedure and

calculate a diffusion coefficient.24-25 The replacement of water with a more complex matrix, such as a homogeneous, thin film likewise introduces complexity into this

measurement, a situation which has also been extensively investigated.26-40 In both

28 simple and complex cases, the most common method used to determine a diffusion

coefficient involves electrochemistry.

The goal of this project is to modify those electrochemical protocols measuring

diffusion coefficients of an ion in a thin film, and apply them to the

spectroelectrochemical sensor. More specifically, films of ionomer poly(vinylbenzyltrimethylammonium chloride) blended with cross-linked poly(vinyl alcohol) or PVTAC-PVA films coated onto indium tin oxide, ITO will be studied with

one of the most common charged particles used in our group, potassium ferricyanide,

K3Fe(CN)6. A diffusion coefficient representing the movement of ferricyanide in a

PVTAC-PVA film will be measured using chronoamperometry.

Experimental

Chemicals and materials. Poly(vinylalcohol) (PVA) (MW 85,000-146,000, 98-

99% hydrolyzed) and 3-aminopropyltriethoxysilane (APTS) were purchased from

Aldrich. Glutaraldehyde (reagent grade, 50% w/w), hydrochloric acid (ACS reagent),

buffers (pH 4, 7, and 10), potassium nitrate, potassium ferricyanide, and sodium

hydroxide were purchased from Fisher. Poly(vinylbenzyltrimethylammonium chloride)

(PVTAC) (30 wt% in water, MW 400,000) was purchased from Scientific Polymer

Products. Stock solutions for polymer blends were prepared by dilution of commercial solutions or by dissolution of solid polymer in deionized water.

Indium tin oxide (ITO)-coated glass (11-50 Ω/sq, ~135 nm thick ITO layer over

Corning 1737F glass) was purchased from Thin Film Devices and diced into 10 mm x 45 mm pieces (slides).

29 Thin Film Preparation. Preparation of PVTAC-PVA films took place according to the procedure described in Chapter 1.

Electrochemical measurements. All electrochemical measurements were performed using a Bioanalytical Systems BAS 100-B electrochemical workstation using a Ag/AgCl reference electrode and platinum mesh auxiliary electrode. An electrochemical cell supporting an ITO slide as working electrode was used and has been previously detailed and because the working electrode is ITO, iR compensation mode

41 was used at all times. Supporting electrolyte used in all cases was 0.1 M KNO3 in distilled water. A step-by-step procedure for the series of electrochemical measurements and calculations used for each film is detailed in Appendix I.

Optical measurements. Film thickness measurements were performed using standard ellipsometric methods on a J.A. Woollam Variable Angle Spectroscopic

Ellipsometer.15 Transmission spectra used to determine absorbance were recorded using

a Hewlett Packard 8453 UV-Visible spectrophotometer.

Results and Discussion

Film saturation. Thin films used in the spectroelectrochemical sensor operate using chemical selectivity dependent upon a charge-based interaction.1,4,8,10 In this

situation, PVTAC-PVA films are positively charged (N+ in the PVTAC structure)

-3 allowing them to attract negatively charged ions such as ferricyanide, Fe(CN)6 . In order

to probe the contents of a thin film, one must first “load” or fill that film’s available

charged sites and this is accomplished by simply exposing the film to the analyte for a

30 period of time. Monitoring this process using cyclic voltammetry allows for a visual

picture of the saturation event (Figure 2.1).

2.0x10-5 2.0x10-5 A

1.5x10-5 1.0x10-5 , A p

-5 0.0 1.0x10 Current, A Cathodic i Cathodic

-6 -1.0x10-5 5.0x10 bare ITO

-5 B -2.0x10 0.0

800 600 400 200 0 -200 -400 0 20406080100

Potential, mV Time, min

Figure 2.1. Saturation of a PVTAC-PVA film coated on ITO and exposed to 0.1 mM

-3 Fe(CN)6 in 0.1 M KNO3. (A) Shows monitoring of the event with cyclic

voltammograms regularly recorded (800 to -300 mV, scan rate 20 mV/s

vs. Ag/AgCl reference). (B) Records cathodic peak current and the

extrapolation of a saturation time.

-3/-4 A regular increase in peak current and appearance of the Fe(CN)6 wave represents

increased analyte in the film. Film saturation takes place in approximately 26 minutes

and is indicated by the repetitive overlap of wave growth and leveling of the saturation

curve (Figure 1B). Similar saturation times have been seen previously (~40 minutes) and

deviation can be attributed to an optimized film recipe for PVTAC-PVA used here along

with heterogeneity from film to film; however, saturation time seen is always on the

order of minutes.12 By then removing ITO coated with loaded film and recording an

absorbance spectrum (versus an unexposed portion of that same film on ITO), one can

calculate the concentration of ferricyanide in the film using Beer’s Law (Equation 2.1)

31 where A is the absorbance value recorded (a.u.), b is the pathlength (film thickness, cm) and c is the concentration (mol/cm3).

Abc=ε (2.1)

In this case with a film thickness of 817 nm, ferricyanide (ε = 1000 M-1 cm-1)25 was found to be 0.48 M in the film, approximately a 5000x increase from solution concentration, indicative of a strong preconcentration factor. This is a much greater factor than previously published data showing an approximate 50x increase and can be attributed to a much more accurate knowledge of film thickness based on recently available ellipsometric measurements.1 Using this concentration, one can calculate the

-3 number of moles (N) of Fe(CN)6 available in the given film volume (known thickness multiplied by the caliper measured area of the film exposed, 2.04 cm2) to give 8.0 x 10-8

-3 moles of Fe(CN)6 available.

Chronoamperometry. Once the film has been loaded, investigation of the behavior of that quantity of analyte can begin. Diffusion coefficients are measured as a rate or distance per unit time. By strictly controlling the time of the experiment and causing a certain portion of analyte to move to the electrode surface (caused by a change in potential), one can calculate a diffusion coefficient (D). Chronoamperometry, or time- based potential stepping allows for such a measurement and equations from F.G. Cottrell

42 -3 may be used for data interpretation. In this case, the Fe(CN)6 loaded film is exposed to supporting electrolyte only, and a chronoamperometric measurement is made (Figure

2.2).

32 0.006 0.006 A 0.005 B 0.005 (1) (1) 0.004 0.004

0.003 (2) 0.003 (2) 0.002 A Current, Current, A (3) 0.002 (3)

0.001 0.001 (4) (4) 0.000 0.000

0.00 0.01 0.02 0.03 0.04 0.05 0 20406080100 Time, s 1/time1/2, s-1/2

Figure 2.2. Chronoamperogram for PVTAC-PVA film on ITO preconcentrated with

-3 0.1 mM Fe(CN)6 . Pulse: 800 to -300 mV, 50 msec width vs. Ag/AgCl

reference. Raw data shown in (A). Cottrell plot necessary for calculation

shown in (B) with Cottrell slope of region (2) only shown as a subset.

-3 By stepping from 800 to -300 mV with a 50 millisecond pulse width, Fe(CN)6 is quickly

-4 -3 reduced to Fe(CN)6 with the limiting factor being how fast Fe(CN)6 can reach the electrode surface. The chronoamperogram shown in Figure 2.2A can be described as a fusion of four general regions (1-4). Initially, a charging current is seen, as quite a bit of energy goes in to charging an electrode to 800 mV in such a short period of time, and this region (1) should be avoided in calculating D. Region (2) most closely represents that time in which the current is controlled by semi-infinite linear diffusion to the electrode and the Cottrell equation might apply. Following this there is a transition period (3)

-3 where more Fe(CN)6 becomes depleted from the thin layer and the transition to thin-

-3 layer electrolysis occurs. This is followed by region (4) where as much Fe(CN)6 that

-4 can be reduced to Fe(CN)6 has done so. When calculating D, it is important to try and

33 get the purest representation of semi-infinite linear diffusion effects, therefore region (2) and its slope is of the most interest.

Obtaining this slope is the most difficult and critical part of the calculation as one must assure that only region (2) is represented, thereby presenting only truly diffusion controlled events. In order to do so, two main factors must be considered. Working electrode ITO slides are notorious for the amount of charging they require due to their poor conductivity and large surface area, therefore in all experiments performed, iR drop which occurs for these reasons, is compensated for by BAS instrumentation by operating under iR compensation mode.43 One must also take into account the amount of charging

44 current necessary for 0.1 M KNO3, used as supporting electrolyte. In a separate experiment, a bare ITO slide was exposed to supporting electrolyte only, and an identical chronoamperogram to that shown in Figure 2.2A was run. By manually subtracting charging current due to 0.1 M KNO3 from the curves shown, one can narrow down region (2), further assuring that the diffusion controlled region is unaffected by charging current (Figure 2.3). This is an acceptable approximation accounting for charging current effects, but it should be noted that charging current will be slightly different on a bare electrode versus a film-coated electrode. Charging current used for 0.1M KNO3 is shown to have a tremendous effect on the chronoamperogram seen in Figure 2.2 and for that reason, it is necessary to work with the current difference between film/analyte and

0.1M KNO3.

34 0.006 0.006 B Film + Analyte Solution A 0.005 0.1 M KNO only 0.005 3 Difference 0.004 0.004 (1)

0.003 0.003 (1) Film + Analyte Solution Current, A

Current, A 0.1 M KNO only 3 0.002 Difference 0.002 (2) (2)

0.001 0.001 (3) (3) (4) (4) 0.000 0.000 0.00 0.01 0.02 0.03 0.04 0.05 0 20406080100 1/2 -1/2 Time, s 1/time , s

Figure 2.3. Chronoamperogram for PVTAC-PVA film on ITO preconcentrated with

-3 Fe(CN)6 (Figure 2.2). Includes effects due to 0.1 M KNO3 in H2O as raw data and the subtracted difference between the two. Pulse: 800 to -

300 mV, 50 msec width vs. Ag/AgCl reference. Raw data shown in (A).

Cottrell plot necessary for calculation shown in (B).

In order to calculate D, a slope must be generated for the diffusion controlled region (2) only using Figure 3B. Complications arise here. Obvious charging current (region 1) can and should be avoided, but one cannot truly separate region (2) from regions (3) and (4) with any level of accuracy. To draw only 10 or 20 data points from this curve and create a section named region (2) is misrepresenting the entire data set. Herein lies the difficulty of extrapolating the diffusion controlled region in a thin film/analyte situation.

Region (2) is essentially clouded in between charging current and a transition period to complete electrolysis within the film, exacerbated by the limited time frame required for the experiment. It is, however, encouraging that the slope of regions (2-4) falls consistently in the range of 10-4 to 10-5 A/s1/2, with greater specificity depending on the number of data points included in the linear regression data set. For the purposes of calculating a diffusion coefficient, this is the first step in understanding that a specific

-3 number describing the movement of Fe(CN)6 in PVTAC-PVA films is potentially

35 unattainable. A more realistic expectation is to find a range of values, or an “effective D” which can generally describe the movement of analyte within the film.

Calculating an effective diffusion coefficient. A diffusion coefficient (D, cm2/s) is described by the Cottrell equation (Equation 2.2) as

nFAD1/2 C m π i= or D= (2.2) π1/2t 1/2 nFAC22 2 2 where n is the number of electrons transferred (in this case 1), F is Faraday’s constant

(96,485 C/mol or A·s/mol), A is exposed, film coated electrode area (2.04 cm2, measured with calipers, but could be more accurately measured electrochemically) 45, and C is

-3 -4 3 concentration of Fe(CN)6 available in the film (4.81 x 10 mol/cm ). The slope (m) of a

Cottrell plot (i vs. t1/2 where plot units must be in A and s) is represented by the slope of the general region (2) as plotted in a Cottrell plot (10-4 to 10-5 A/s1/2, Figure 2.3B). An

-3 effective diffusion coefficient representing the movement of Fe(CN)6 in a PVTAC-PVA film was calculated to fall in the region of 10-11 to 10-13 cm2/s, orders of magnitude

-3 -6 slower than that of Fe(CN)6 in free solution with an ITO working electrode (3.9 x 10 cm2/s, calculated according to established procedures).46

To confirm the accuracy of this value, one can use the Einstein equation to generally estimate a film loading time (t) based on D and film thickness (l) or the maximum distance able to be traveled within the film (Equation 2.3)47-48.

l2 = t (2.3) 2D⋅

Based on D calculated above, a general loading time is calculated on the order of minutes. Given an effective D, this value can vary as low as seconds and as high as several hours, but given the roughness of the calculation, obtaining the same general

36 order of magnitude in loading time (~26 min) is significant. Error can be attributed to three factors in particular: (1) this is a very general equation and is useful as a rule of thumb, but not for an exacting calculation in this situation, (2) (l) is the standard deviation of a Gaussian curve and represents a distance traveled by an “average” molecule and should not be considered to be absolute, and (3) an effective D is used.

Diffusion control can be confirmed with a scan rate study and is based upon the work of Randles and Sevcik.27,49 Peak current is dependent upon scan rate, with diffusion controlled analytes exhibiting non-linear dependence, as is this case (Figure 2.4).

1.5x10-3 A 1.2x10-3 1.0x10-3 B 1.0x10-3

5.0x10-4 8.0x10-4 , A p

0.0 -4 6.0x10 Current, A Cathodic i Cathodic

-4 -5.0x10 4.0x10-4

-4 -1.0x10-3 2.0x10

0.0 -1.5x10-3 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 800 600 400 200 0 -200 -400 Potential, mV Scan Rate1/2, V/s1/2

Figure 2.4. Randles-Sevcik plot for a PVTAC-PVA film loaded with 0.1 mM

-3 Fe(CN)6 . Data points polynomially fitted (B) shown are derived from

cyclic voltammograms at 0.001, 0.005, 0.025, 0.050, 0.10, 0.25, 0.50,

0.75, 1.0, 5.0, and 10.0 V/s (A), all vs. Ag/AgCl reference.

An interesting test can also be performed by integrating the area under the curve of the 50 millisecond chronoamperogram, or producing a chronocoulogram. The value obtained represents the charge present in the film during the potential step (Q, =5.2 x 10-5

37 Coulombs as obtained using BAS software) and one can use this to calculate moles of

-3 50 Fe(CN)6 reduced during that time (N) (Equation 2.4) . Q =N (2.4) nF

Again, n, is the number of electrons transferred (1) and F is Faraday’s constant. In the 50 millisecond potential step, only 5.39 x 10-10 moles of ferricyanide are converted out of an available 800 x 10-10 or less than 1%, an extremely small fraction. This is unsurprising for such a short amount of time, and begs the question, what would the effects be of a longer potential step experiment?

Longer potential step evaluation. For these reasons, the film was re-evaluated using a 32 second step, the maximum allowed by the instrumentation without manual programming (Figure 2.5).

3.5x10-4 3.5x10-4 3.0x10-4 B 3.0x10-4 A 2.5x10-4 2.5x10-4 (1) (1) -4 2.0x10 2.0x10-4

1.5x10-4 -4 1.5x10 A Current, Current, A 1.0x10-4 1.0x10-4 (2) 5.0x10-5 (3) 5.0x10-5 (2) (3) (4) (4) 0.0 0.0

-5 0 5 10 15 20 25 30 35 0123456 1/2 -1/2 Time, s 1/time , s

Figure 2.5. Chronoamperogram for PVTAC-PVA film on ITO preconcentrated with

-3 Fe(CN)6 . Pulse: 800 to -300 mV, 32 sec width vs. Ag/AgCl reference.

Raw data shown in (A). Cottrell plot necessary for calculation shown in

(B).

38 In the unconverted chronoamperogram, immediately visible is the lack of separation between regions 2-4. Charging current (1) is still present and must be discounted in the

-3 calculation of D, but given a much longer time frame, the region in which Fe(CN)6 is traveling to the electrode surface is elongated, and in fact represents diffusion control for a greater number of charged particles moving in the film. By calculating this number (N) as shown in Equation 2.4 with a recalculated value for integrated charge (Q = 2.58 x 10-4

-10 -3 C), one finds that indeed, 26.7 x 10 moles of Fe(CN)6 is being converted, or 3% of the total 800 x 10-10 moles available. (While this is certainly more than that converted in the

50 millisecond step, it would be interested to investigate the potential step pulse width, or time required to convert a majority of those moles available. Additionally, not all

-3 Fe(CN)6 in the film may be “available” for conversion, as some may be irreversibly bound or trapped in portions of the film.)16,20-22,51 It is still critical to separate out the diffusion controlled region as was shown for the 50 millisecond step and this manual subtraction of 0.1 M KNO3 charging current is found in Figure 2.6.

1.5x10-4 3.5x10-4 A Film + Analyte Solution 0.1 M KNO only B Film + Analyte Solution 3 3.0x10-4 0.1 M KNO only Difference 3 Difference -4 1.0x10 -4 2.5x10 (1) 2.0x10-4

Current, A Current, -4

-5 (1) Current, A 1.5x10 5.0x10 (2)

1.0x10-4 (2) (3) (3) (4) 5.0x10-5 (4) 0.0 0.0 0 5 10 15 20 25 30 35 0123456 Time, s 1/time1/2, s-1/2

Figure 2.6. Chronoamperogram for PVTAC-PVA film on ITO preconcentrated with

-3 Fe(CN)6 (Figure 2.5). Includes effects due to 0.1 M KNO3 in H2O as raw data and the subtracted difference between the two. Pulse: 800 to -

39 300 mV, 32 sec width vs. Ag/AgCl reference. Raw data shown in (A).

Cottrell plot necessary for calculation shown in (B).

The Cottrell plot shown in Figure 2.6B can be used to again give a general slope. It can be seen that 0.1 M KNO3 charging effects play a slightly smaller role in the longer time frame case, but still, region (2) is somewhat obscured by regions (1), (3), and (4) and a general slope using all regions, but avoiding (1) is found to be on the order of 10-4 to 10-5

A/s-1/2. Based on this data, D is calculated as in Equation 2.2 and is again found to fall between 10-11 and 10-13 cm2/s, remarkably similar to that found for the 50 millisecond step!

Conclusions

Setting out in this series of experiments, it was meant that a steadfast value for the

-3 diffusion of Fe(CN)6 in a PVTAC-PVA film be determined and used as a reference for others encountering this or similar situations. What has been presented in this chapter is essentially a case study of one PVTAC-PVA film undergoing a series of several experiments, the method of which was carefully developed (Appendix I). What has not been presented is the countless number of repetitions of this same process on other

PVTAC-PVA films. In nearly every case, the experience was quite similar. A variety of

D values were found; however, in all cases, including the case study above, these values fell in a relatively small range on the order of 10-11 to 10-13 cm2/s. It seems that one value for D is simply not in the cards! What is reassuring though, is that this variance can be addressed, and is, at least, a consistent variance. It seems that an “effective D” is being measured with its value depending heavily on film structure and homogeneity. Each film

40 that is made is unique, particularly in the case of polymer-based films. These

-3 irregularities can present a different matrix for Fe(CN)6 on the microscale level, and it essentially moves at slightly different rates, not only comparing film to film, but also within the same film! Electrochemical measurements of this rate are simply composite

-3 values of a not-so-widely ranging Fe(CN)6 movement pattern. This idea is supported by findings within the group that suggest convoluted film matrices with channels and pores of varying sizes, wherein particles move at different rates and can even become trapped or block a path..16,20-22,51 Additionally, calculated D values found in literature for widely varying matrices and analytes fall in a similar range (10-10 to 10-14 cm2/s) and refer to an

“apparent diffusion coefficient” in several cases.26-40

Several calculations of D were performed on other films used in this group including sol-gel processed silica hosts containing either the ionomer poly(diallyldimethylammonium chloride) or quaternized poly(vinyl pyridine) (sol-gel

PDMDAAC and sol-gel QPVP). While these tests were not as rigorous as those presented for PVTAC-PVA it was found that again, calculated D varied within the range of 10-11 to 10-13 cm2/s. All of these experiments focused on cathodic reduction of

-3 Fe(CN)6 . Several calculations were made based on anodic peak currents and therefore,

-4 the diffusion coefficient of the reduced species Fe(CN)6 . It seems that the charge of the diffusing species in this case played little role in its rate of movement and D values still fell in the range of 10-11 to 10-13 cm2/s.

-3 Calculating a single diffusion coefficient for Fe(CN)6 in thin films used as part of the spectroelectrochemical sensor is simply unreasonable. Inherent in the use of these films are variables that cannot be regulated to the extent a single D value would require.

41 Additionally, the extrapolation of a diffusion controlled region only from collected data is difficult and a balancing act between narrowing down a selection and giving a true representation of an entire data set, all acts performed within the time limits necessitated during experiments. It is also important to remember that confirmation of this value used only electrochemically based methods, and an independent method of measuring D such as that described by T. Imato et al. should be considered.52

Nonetheless, understanding the movement of analytes within these films though quite complex, may be addressed. The rate at which species move is variable, but should generally fall in the region of 10-11 to 10-13 cm2/s given that the analyte is similar to that

-3 of Fe(CN)6 (A significantly different analyte/film system will require a similar study to that presented). This rate of diffusion is orders of magnitude slower than that seen in solution alone (on the order of 10-6 cm2/s) and represents the difficulties encountered when an analyte travels in a more complex matrix, such as a thin film. And thus, a quasi- conclusion to the great diffusion coefficient quest is found.

42 Chapter 3.

-4 Hybrid Optically Transparent Electrodes and Fe(CN)6

Introduction

Optically transparent electrodes (OTE) were born of a desire to spectroscopically observe electrochemical processes occurring at an electrode surface. R. N. Adams commented as such, and saw the desirability of having a conductive surface that was also optically transmissive.53 The advent of the OTE thus was important in the development of spectroelectrochemistry, which rapidly became a useful analytical technique. The body of research now devoted to or employing this technique is vast. Useful applications of spectroelectrochemistry include study of reaction kinetics, reaction mechanisms, identification of reaction intermediates, stoichiometry, number of electrons transferred, standard reduction potentials, molar extinction coefficients, and diffusion coefficients.53

The choice of an appropriate OTE to obtain such information is critical and is typically bound by both optical (i.e., transmissive to a particular region of the electromagnetic spectrum) and electrochemical properties such as (1) wide potential window, (2) low resistivity, and (3) sufficiently facile heterogeneous electron exchange to accommodate a wide range of redox processes. The first OTE consisted of a glass substrate coated with a thin transparent film of antimony-doped tin oxide.54-55 Since then a variety of OTEs have been developed providing a wide range of properties for different applications. Most OTEs consist of a transparent substrate (glass or quartz) coated with a thin metallic film such as platinum56, gold57-60, or germanium for infrared optical windows.61-62 Thin wire meshes (mini-grids) have also been employed.63 Carbon films

43 have been vapor-deposited onto both glass and quartz, providing an adhesion layer upon which a mercury transparent film electrode is formed.64 Polyester sheets have even been used as a more flexible substrate and are coated with thin films of metals or metal oxides.65 All of these OTEs share a reasonably wide optical window (near ultraviolet to infrared) with an equally reasonable potential range (ca. -1.4V to +1.2 V); however, they are limited by tradeoffs between conductivity and transparency. Metal films must be thin enough to be optically transparent, but such films can be relatively resistive, giving rise to electrochemical signals distorted by IR drop. Additionally, these films can be quite fragile, and the expense and difficulty of their preparation has proven to be problematic.66-69

OTEs have also been constructed by coating a glass substrate with a thin film of

“doped” oxide, such as tin oxide or indium oxide.70 Additionally, indium oxide may be doped with tin, producing a thin layer of indium tin oxide (ITO). ITO thin films are of great interest due to their high transparency and good electrical conductivity.

Applications are wide ranging and include use in solar cells, liquid crystal and flat panel displays, EMI/RFI shielding, photo detectors, and increasing use in electroanalytical and spectroelectrochemical sensors.71-80 In addition to desirable optical and electronic properties, ITO offers a well-established manufacturing process resulting in rugged thin films of ITO on transparent substrates.78 However, despite all of these advantages as

OTEs, ITO does not provide the level of conductivity found in some thin metal films and is not responsive to all redox species. It would therefore be desirable to combine the advantages of both thin metal films and ITO to produce a hybrid film that is thin, optically transparent, conductive, and durable enough to endure rugged experimental use.

44 Hybrid substrates of this nature have thus far been developed on a limited basis.

Wachter, et al. coated quartz substrates with thin metal films followed by a protective cover of SiO2 (surface activated with various components), all applied to the entire surface of the substrate to increase durability.81 Additionally, ITO capped with a variety of nanometer thick metal or oxide buffer layers has been investigated as a hybrid component of an organic light-emitting diode.82-84 To date, however, there have been no reports of these or similar hybrid films being used as OTEs for spectroelectrochemical applications.

Several novel hybrid OTEs for use in electroanalytical and spectroelectrochemical sensing have been developed.41 Commercially available ITO glass substrates were coated with an additional thin layer of gold, platinum, palladium, or carbon via thermal evaporation or sputtering. With the combination of both ITO and a thin conductive layer,

OTEs were produced that exhibit good electrical conductivity, optical transparency, and increased durability. By combining ITO and thin conductive layers, an established manufacturing process (i.e., commercial production of ITO glass) was added to thin conductive layers to enhance the electrochemical performance of the original substrate

(ITO glass) at a minimal cost to optical transparency.

Described herein is the electrochemical characterization of hybrid OTEs of gold-

ITO, carbon-ITO, platinum-ITO, and palladium-ITO which were previously constructed.

Gold, carbon, and platinum were chosen due to their extensive use in the field of electrochemistry, and palladium was readily available during hybrid production.

Electrochemical characterization is provided for the common electrochemical redox probe potassium ferricyanide, K3(Fe(CN)6). Throughout all characterization procedures,

45 performances of the hybrids are compared to those of their components, namely bare ITO glass and commercially available metal/carbon disk electrodes.

Experimental

Reagents. Potassium ferricyanide (Aldrich) was used as a redox probe.

Potassium nitrate (Aldrich) was employed as supporting electrolyte at 0.1 M in distilled water (Barnstead water system: Dubuque, IA). Indium tin oxide (ITO) coated 1737F glass was obtained from Thin Film Devices (Anaheim, CA) as 14 x 14 in. sheets and diced into 10 x 45 mm or 1 x 3 in. sections (slides). The manufacturer claims a 150 nm

ITO thickness with a sheet resistance of 20 Ω/ (nominal values). Slide cleaning was performed using methanol (Fisher).

Electrochemical Measurements. For all electrochemical measurements of hybrid film OTEs, a standard procedure was established and followed with potassium ferricyanide at a concentration of 1.0 mM in 0.1 M KNO3. Solutions were thoroughly deoxygenated by nitrogen bubbling prior to making any electrochemical measurements.

All electrochemical measurements were performed using a Bioanalytical Systems

BAS 100-B electrochemical workstation. A specialized electrochemical cell was constructed using a 28 x 57 mm disposable glass vial (Fisher) serving as solution container for 22 mL of the analyte/electrolyte solution to be investigated. A specialized cap for this vial was machined with three holes (2 rectangular, 1 circular) for the insertion of varying electrodes. Platinum mesh inserted into one side of the cap served as the auxiliary electrode. A Ag/AgCl, 3 M NaCl reference electrode (Bioanalytical

Systems) was inserted into the central, circular hole of the cap, and the remaining hole

46 was reserved for the hybrid electrode to be investigated as the working electrode. For cases in which glassy carbon, gold, or platinum disk electrodes (all sourced from

Bioanalytical Systems) served as working electrodes, a secondary cap/vial system was used wherein 3 circular holes are present. In order to standardize the surface area of each electrode exposed to solution, placement of the electrodes in the cap was carefully controlled. The working electrode was placed in the cap with its electrochemically active surface facing the center of the vial (toward the reference electrode). An O-ring was placed 8 mm from the top of the slide to secure its position. Contact was made with an alligator clip at the top of the slide over a piece of copper foil serving as a protective layer for the hybrid surface. The bottom of the platinum mesh auxiliary electrode was made level with the bottom of the working electrode, and the frit of the Ag/AgCl reference electrode extended below the base of the hybrid film electrode. Additionally, a standard cleaning procedure for the hybrid electrodes was used. Each underwent four rinses with no scrubbing or wiping (Isoclean soap in distilled water, distilled water, methanol, distilled water) followed by careful blotting with lens paper before insertion into the cell assembly.

A standard set of cyclic voltammograms was performed with each hybrid electrode. Each voltammogram was run both with and without positive feedback IR compensation as provided by the BAS 100B potentiostat. A voltammogram of the usable potential window was determined in supporting electrolyte using BAS stationary voltammetry electrodes (either glassy carbon, platinum, or gold disk electrode depending on the composition of the related hybrid; GC for carbon/ITO hybrid, Pt for Pt/ITO hybrid, etc.) at a scan rate of 20 mV/s. Identical voltammograms were subsequently obtained

47 using each hybrid. Next, potassium ferricyanide solution was employed, as voltammograms were obtained using hybrid electrodes and repeated on a bare ITO surface. For each hybrid, a potential sweep rate study was also completed spanning sweep rates from 5 to100 mV/s in potassium ferricyanide.

Results and Discussion

Establishing a working potential window. Figure 3.1 illustrates the accessible potential windows in 0.1 M KNO3 for each type of hybrid electrode. Predictably, none of the hybrids provides as featureless or as wide a potential window as ITO, whose useful range spans from ca. 1.4 V to –1.0 V. Hybrid potential limits appear similar to those of the corresponding bulk metal or carbon electrodes, which in turn are considerably more electrochemically active than ITO. For example, the Au hybrid offers a relatively featureless negative potential window, but positive scanning results in characteristic oxide formation on Au and its subsequent reduction on the return sweep. In contrast, the Pt hybrid electrode offers a wide positive window, but the negative potential window is hampered by hydrogen evolution. Similar behavior to Pt was seen with the Pd hybrid, as well. The carbon hybrid provided a clean positive window, but the onset of a reductive wave by ca. –400 mV (vs. Ag/AgCl) appears to limit the negative window relative to glassy carbon. Of particular note is the increased activity of the hybrid electrodes relative to their bulk metal/glassy carbon counterparts, as evidenced by larger capacitive charging envelopes and enhanced faradaic currents associated with surface processes (i.e., oxide formation (Au) and gas evolution (Pt and Pd)).

48 0.2 A

0.0

ITO Au -0.2 Au/ITO

0.2 B 2

0.0

mA/cm ITO GC -0.2 C/ITO

0.2 C

Current density, 0.0

ITO Pt -0.2 Pt/ITO

D 0.0

-4.0 ITO Pd/ITO -8.0 1500 1000 500 0 -500 -1000 Potential, mV

Figure 3.1. Accessible potential windows in 0.1M KNO3 for each of four hybrid electrodes: (A) Au/ITO, (B) C/ITO, (C) Pt/ITO, and (D) Pd/ITO.

These differences are especially apparent given that currents in Figure 3.1 are normalized for electrode area. This observation concurs with the idea that thin metal

49 films may actually be an accumulation of microscopic island features thereby increasing electrochemically accessible surface area (i.e., the microscopic surface area).

-3 Cyclic voltammetry using hybrids with FeCN6 . Each type of hybrid electrode was further evaluated by obtaining cyclic voltammograms of the well understood redox species potassium ferricyanide in 0.1 M KNO3 electrolyte. For comparison purposes, voltammograms were obtained in the same solutions using bare ITO electrodes, bulk Pt and Au disk electrodes, and a glassy carbon disk electrode. These data are summarized in

Figure 3.2 and in Table 3.1. All voltammograms were obtained using positive-feedback iR compensation to eliminate the effects of cell and electrode uncompensated resistance.

Figure 3.2 is a comparison of voltammograms for all hybrid electrodes (and bare

ITO) obtained in a 1 mM solution of potassium ferricyanide. Panel A contains data obtained using thin, medium, and thick hybrid Au films on ITO.41 All of the voltammograms display relatively facile electron transfer kinetics, with peak potential differences (∆Ep) ranging from 140 – 160 mV (Table 3.1). Interestingly, Au film thickness played little role in voltammetric response, with all Au films yielding voltammograms similar to those obtained on bare ITO. This observation is attributed to the conductivity of the underlying ITO layer, as well as the favorable nature of redox processes at Au surfaces.

50 200 A 200 B

A 100 A 100 µ µ

0 0

Current, Current, bare ITO Current, bare ITO -100 thin Au/ITO -100 thin C/ITO medium Au/ITO medi um C/ ITO thick Au/ITO thick C/ITO -200 -200 800 600 400 200 0 -200 -400 1000 500 0 -500 Potential, mV Potential, mV

300 400 C D 200 300

A A 200 µ 100 µ 100 0 Current, Current, Current, 0 bare ITO bare ITO -100 thin Pt/ITO -100 thin Pd/ITO thick Pt/ITO thick Pd/ITO -200 -200 800 600 400 200 0 -200 -400 800 600 400 200 0 -200 -400

Potential, mV Potential, mV

-3 Figure 3.2. Cyclic voltammograms for Fe(CN)6 on ITO hybrid and bare ITO

electrodes (1.0 mM in 0.1M KNO3, vs. Ag/AgCl, 20 mV/s): (A) Au/ITO,

(B) C/ITO, (C) Pt/ITO, and (D) Pd/ITO.

∆Ep (mV) ipc/ ipa Au Thin 140 0.89 Medium 150 0.86 Thick 150 0.94 Au Disk 160 0.95

C Thin 1050 4.18 Medium 600 10.85 Thick 440 1.73 GC Disk 180 1.16

Pd Thin 160 1.46 Thick 310 1.60

Pt Thin 80 2.51 Thick 140 1.55 Pt Disk 100 1.76

ITO 160 0.99

Table 3.1. Peak current potential difference and cathodic/anodic peak current ratios

for potassium ferricyanide (K3FeCN6) coupled with all working electrodes

studied.

For carbon films on ITO, shown in panel B, all voltammograms exhibit larger ∆Ep values than bare ITO, indicating sluggish electron transfer kinetics at the carbon film surface.

Notably, ∆Ep values decreased as carbon film thickness increased. This result is unexpected given that the thin film would be considered to be most similar to that of the underlying ITO layer. Electron transfer kinetics observed at Pt and Pd hybrid electrodes

52 are similar to those at bare ITO, but hydrogen evolution interferes with the measurement of cathodic peak currents, especially in the case of thick films.

The next phase of investigation involved a sweep rate study of hybrid electrodes in ferricyanide solutions, the results of which may be seen in Figure 3.3. For all hybrid electrodes, thin and medium layers of metal or carbon behave quite similarly, with

Au/ITO showing the most facile and reversible electron transfer. Both C/ITO and Pt/ITO display reversibility scan rate dependence with lagging anodic peak current values in the reverse step. Deviation in C/ITO and Pt/ITO anodic peak currents may be attributed to estimation of peak current values from the original cyclic voltammogram at fast scan rates. Pd/ITO hybrid electrodes were not available for this study.

600 A

400

200 thin Au/ITO 0 medium Au/ITO thick Au/ITO -200

-400

-600 0.05 0.10 0.15 0.20 0.25 0.30 0.35

350 B 300

A 250

µ 200 150

100 thin C/ITO medium C/ITO 50 thick C/ITO 0 -50 -100 0.05 0.10 0.15 0.20 0.25 0.30 0.35

Peak Current, 600 C

400

200 thin Pt/ITO thick Pt/ITO 0

-200

-400 0.05 0.10 0.15 0.20 0.25 0.30 0.35

Scan Rate, V/s-½

Figure 3.3. Sweep rate study showing cathodic and anodic peak currents for

potassium ferricyanide coupled with each of three hybrid electrodes: (A)

Au/ITO, (B) C/ITO, and (C) Pt/ITO.

54 Conclusions

It has been demonstrated that thermally deposited films of Au, C, Pt, and Pd on

ITO-coated glass substrates can function favorably as optically transparent conductive surfaces upon which spectroelectrochemical investigations can be carried out. Thin film hybrid electrodes should be acceptably transparent for many applications, especially when the thinnest films are employed. The real advantages of these films become apparent from electrochemical responses obtained for potassium ferricyanide and other redox probes41, which are comparable to or improvements over those obtained at bare

ITO. In most cases (i.e. noble metals) the thinnest and most optically transparent films yielded the most favorable voltammetric responses in terms of facile electron transfer kinetics.

For most routine spectroelectrochemical applications, the Au thin film hybrid electrodes should provide the best performance due to their superior optical transparency and favorable voltammetric responses for all redox probes tested. Other hybrid films also behaved favorably and should suffice in more specialized situations as called for by the analyte.

55 Chapter 4.

4-vinyl-4´-methyl-2,2´-bipyridine as a Ligand for Use in the

Spectroelectrochemical Sensor.

Introduction

During the development of a spectroelectrochemical sensor for pertechnetate,

- TcO4 , it became necessary to investigate ligand incorporation in the sensor film structure

- due to complex and non-ideal electrochemical and optical properties of TcO4 which neither absorbs nor emits in the near-UV/visible region of the spectrum.85-86 In early tests

- with the sensor design, TcO4 preconcentrated into films, but its electrochemistry was poorly defined. Additionally, upon electron transfer, no obvious optical or fluorescence change was detected, thereby complicating, if not eliminating spectroscopic modulation as a means of detection.86 An approach with some modification was necessary. It is thought that with the addition of a binding ligand to the film, a structure might be

- provided in which TcO4 would not only preconcentrate, but undergo more defined electrochemical cycling accompanied by some spectroscopic change.85

- Due to the nature of working with TcO4 , it is necessary to choose a ligand which can be used in non-radioactive proof-of-concept experiments. 4-vinyl-4´-methyl-2,2´- bipyridine or vinylbipyridine, vbpy, has been used as a complexing ligand with several metals including ruthenium, iron, osmium, iridium, and zinc and was interesting both to us and our collaborators at Pacific Northwest National Laboratories.87-92 In these cases, the metal+2-ligand complex was electropolymerized onto a substrate and some preliminary spectroscopic studies were completed. Vinylbipyridine complexed with zinc

56 became especially interesting based on the work of Meyer, Sullivan, and Caspar.92

+2 Following electropolymerization of a Zn(vpby)3 complex onto an electrode surface,

+2 +2 exposure to FeCl2 (aq) resulted in the competitive displacement of Zn metal by Fe , and

+2 the subsequent formation of Fe(vbpy)3 films. This mechanism reflected the general

- templating approach desired in the development of the TcO4 sensor. An unknown factor was the limited nature of spectroscopic studies on these films concerning the Zn+2/Fe+2 exchange.

According to conversations with Dr. B. Patrick Sullivan (University of

+2 Wyoming), initially synthesizing and working with Fe(vbpy)3 was simpler and more straightforward and could provide a knowledge baseline of sorts to work from prior to

+2 +2 +2 attempting the Zn(vbpy)3 electropolymerization and Zn /Fe exchange. Given this

+2 +2 framework, the approach taken was to synthesize both Fe(vbpy)3 and Zn(vbpy)3 and complete basic absorbance and fluorescence studies of each complex.

+2 Electropolymerization of Fe(vbpy)3 films was to be completed first, followed by

+2 +2 +2 electropolymerization of Zn(vbpy)3 films. The exchange between Zn and Fe would then be attempted, with optical monitoring via reflectance spectroscopy.

This proof-of-concept experiment gives a rough outline for an approach that may

- eventually be taken with TcO4 - vbpy based chemistry, perhaps with a rhenium-vbpy

+2 complex serving as the template in a role similar to that of Zn(vbpy)3 .

Experimental

Chemicals and materials. Ferrous chloride (Sigma), ammonium hexafluorophosphate (Acros), zinc chloride and diethyl ether (Fisher) were used without

57 further purification. 4-vinyl-4´-methyl-2,2´-bipyridine, vbpy, was supplied by collaborators at Pacific Northwest National Laboratory (Richland, WA). Tetra-n- butylammonium hexafluorophosphate, tBAPF6, was made by the Connick group at the

University of Cincinnati according to an established laboratory procedure based upon that of C.M. Elliott, et al.93 Part of that procedure included two recrystallizations from methanol and a third from methylene chloride/diethyl ether (Fisher). The material was then dried in a vacuum oven prior to use. Solvents supplied by Tedia Company, Inc.

(Fairfield, OH) include acetone, acetonitrile (CH3CN), methanol, and methylene chloride all used without further purification with the exception of CH3CN, which was distilled over calcium hydride (Acros). Compressed nitrogen gas was obtained through Wright

Brothers, Inc. (Cincinnati, OH).

Synthesis of Fe(vbpy)3(PF6)2. Tris(4-vinyl-4´-methyl-2,2´-bipyridine)iron(II),

+2 Fe(vbpy)3 , was prepared with P.J. Ball (Connick Group, University of Cincinnati) according to C.M. Elliott, et al.91 Characterization of the complex was performed using elemental analysis (Atlantic Microlab, Inc., Norcross, GA) and 1H NMR (Figure 4.1).

Elemental analysis shows that two residual waters were not removed in the drying process (Calc: C, 50.12; H, 3.88; N, 8.99. Found: C, 48.48; H, 3.76; N, 8.65.).

Synthesis of Zn(vbpy)3(PF6)2. Zn(vbpy)3(PF6)2 was prepared according to T.J.

Meyer, et al.92 in a double batch and the structure confirmed by elemental analysis and 1H

NMR (Figure 4.1). In this case, elemental analysis reveals that no waters remain in the structure (Calc: C, 49.62; H, 3.84; N, 8.90. Found: C, 48.98; H, 3.86; N, 8.78.).

It is important to note that in both synthesis cases, this data represents a typical result, but several batches of each compound were made with equal success and

58 structures were always confirmed by 1H NMR and often using elemental analysis. Water remaining in the structures varied between 1 and 3 molecules, as confirmed by elemental analysis, and it is important to note that water was more easily removed from the

+2 Zn(vbpy)3 compound.

Hybrid Electrodes. Hybrid electrodes used throughout the course of these experiments are those described in detail in Chapter 3.

59 g h i,j c d b e N N a f

M+2

solvent g

a-f h-j

8 6 4 2 0 PPM Zn(vbpy)3(PF6)2 solvent g

a-f h-j

8 6 4 2 0 PPM Fe(vbpy)3(PF6)2

1 +2 Figure 4.1. H NMR spectra of Fe(vbpy)3(PF6)2 and Zn(vbpy)3(PF6)2. General M -

ligand structure is shown with assigned peaks. Solvents are water,

acetonitrile, and tetramethylsilane reference.

60 Electrochemical measurements. All electrochemical measurements were performed using a Bioanalytical Systems BAS 100-B electrochemical workstation and a standard electrochemical vial sealed with parafilm, as described previously.41 Solutions were deoxygenated for a minimum of thirty minutes prior to experimentation using N2 gas bubbled through distilled CH3CN. A quasi Ag/AgCl reference electrode was used (a length of silver wire dipped in Clorox® for 2-3 minutes) and platinum wire or mesh served as the auxiliary electrode. Supporting electrolyte used in all cases was 0.1 M tBAPF6 in distilled CH3CN.

Optical measurements. Solution absorbance and fluorescence measurements were made using a Hewlett Packard 8453 UV-Vis spectrophotometer and Cary Varian

Eclipse fluorimeter, respectively. Optical microscopy was performed using an inverted

Nikon Epiphot microscope. Reflectance measurements were recorded using modification to existing fluorescence instrumentation.17 In place of a sample cell, the Pt disk electrode was sturdily mounted using a post and clamp system. Light sources used were as follows: xenon arc lamp for absorbance and HeCd laser, λ=325 nm, for fluorescence.

Existing optical fibers for light source supply and absorbance detection were replaced with quartz core SMA optical patch fibers with a core diameter of 400 µm (Ocean Optics,

Inc., Dunedin, FL). A WG 345 filter (Esco Products, Inc., Oak Ridge, NJ) was placed before the monochromator entrance slit in order to eliminate the 325 nm laser line.

Specific physical details for instrumentation at the reflectance and collection point are illustrated in Figure 4.2.

From light source

Pt disk electrode B

To monochromator C

Figure 4.2. Bird’s eye view of physical set-up used for reflection based

measurements. (A) and (C) represent quartz core optical fibers used with

collimators. (B) is a 6-fiber bundle fluorescence detection cable. Fibers

are aligned such that the light source (fiber A) is directed onto the Pt core

of the disk electrode and the specular reflection is collected by fiber C.

Results and Discussion

Spectroscopy. Synthesis of both Fe(vbpy)3(PF6)2 and Zn(vbpy)3(PF6)2 followed fairly straightforward procedures, but there was a lack of success in consistently

+2 removing all residual water molecules from the product, particularly in the Fe(vbpy)3 complex. Vacuum drying for up to 4 weeks was attempted; however, due to the

62 reactivity of the vinyl group and the possibility of thermal polymerization, no extensive heating could be used to drive off water. Its presence could be a factor in the electropolymerization of both materials.

Initial absorbance and fluorescence studies of each compound in solution, along with precursor vinylbipyridine alone, were completed. Absorbance spectra (Figure 4.3) show strong absorbance in the UV region by the vbpy ligand, with a slight red shift for both the Fe- and Zn- complexes, indicative of complexation with a metal.

2.5 vbpy Fe(vbpy) +2 2.0 3 Zn(vbpy) +2 3 FeCl 1.5 2

1.0 Absorbance, AU

0.5

0.0

200 300 400 500 600 700 Wavelength, nm

+2 +2 Figure 4.3. Absorbance spectra for 0.01mM vbpy, Fe(vbpy)3 , Zn(vbpy)3 , and

FeCl2 all solutions in 0.1M tBAPF6/CH3CN.

63 +2 Of particular note is the absorbance peak for Fe(vbpy)3 at λ=540 nm which occurs at a

+2 baseline for vbpy, Zn(vbpy)3 , and FeCl2 and is indicative of a wavelength able to be used for monitoring in the visible region during a Zn+2/Fe+2 exchange.

+2 Fluorescence studies were performed for all compounds with only Zn(vbpy)3 showing fluorescence emission (Figure 4.4). Excitation maxima fall between 250-300 nm and are comparable to the absorbance profile shown in Figure 4.3 (Absmax range ≈

240-290 nm).

140

λ = 250 nm 120 ex λ = 307 nm ex 100 λ = 290 nm ex 80 Excitation Emission 60 Intensity, a.u. λ = 310 nm 40 ex

200 300 400 500 600 700 800 W avelength, nm

+2 Figure 4.4. Fluorescence spectra of 0.005mM Zn(vbpy)3 in 0.1M tBAPF6/CH3CN.

Excitation was performed at several wavelengths and the consistent emission profile

+2 shown by Zn(vbpy)3 is indicative of complex purity (λmax = 339 nm). The second order

64 +2 of the Zn(vbpy)3 profile is present between 640 and 750 nm. Peaks at 500, 580, 614, and 620 nm for double the excitation wavelength are also visible. Again, a Zn+2/Fe+2 exchange can be monitored spectroscopically at λmax = 339 nm where only the Zn complex fluoresces.

+2 Electropolymerization. Electropolymerization of Fe(vbpy)3 was first performed on a platinum disk electrode, the most common electrode used in the literature.87-92 The reaction is extremely sensitive to the presence of oxygen, as electron hungry radicals may initiate polymerization of the vinyl group on vbpy in solution, not on the electrode surface as desired. It was necessary to completely seal the vial in which the experiment was being performed with several layers of parafilm and deoxygenate the solution for a minimum of thirty minutes. If deoxygenation was insufficient, reduction and oxidation peaks for O2 would appear at approximately -1.0 and 0.0 V, respectively, overshadowing those of the vbpy. Once deoxygenated, the potential was scanned repeatedly between -

0.8 and -1.75 V. Increasing waves show the redox couple of vbpy and represent growth of the film (Figure 4.5A). Once the film is in place on the Pt disk electrode, it is

+2 removed from the Fe(vbpy)3 solution, rinsed, and placed in supporting electrolyte only

(0.1M tBAPF6/CH3CN). A larger potential window scan takes place and the presence of the film is indicated by the Fe+3/+2 redox peak appearing at +1.0 V along with the vbpy couple in its original location (Figure 4.5B). A reddish-orange film is visible to the naked eye on the Pt electrode surface and appears to be mechanically stable to rinsing with both aqueous and non-aqueous solvents. Visual confirmation of film presence is seen with before and after images (Figure 4.6). A mottled film appears to have formed fairly uniformly across the Pt surface with little or no extension onto the surrounding

65 Teflon insulation. A few spots of electrode surface that have not been coated appear as small pinholes in the film, the overall structure appears to be free of gross defects.

1.2x10-4 8.0x10-5 A 1.0x10-4 B 6.0x10-5 8.0x10-5 4.0x10-5 6.0x10-5

2.0x10-5 4.0x10-5 Current, A Current, A Current, 0.0 2.0x10-5

-2.0x10-5 0.0

-2.0x10-5 -4.0x10-5

-4.0x10-5 -800 -1000 -1200 -1400 -1600 -1800 1500 1000 500 0 -500 -1000 -1500 -2000 Potential, mV Potential, mV

+2 Figure 4.5. Electropolymerization of 3mM Fe(vbpy)3 in 0.1M tBAPF6/CH3CN vs.

quasi Ag/AgCl reference at 100 mV/s (A), and investigation of the

+2 resultant Fe(vbpy)3 film on a Pt disk electrode in supporting electrolyte

only (B). Both cyclic voltammograms include a scan using a bare Pt disk

electrode in supporting electrolyte only.

A B

Figure 4.6. BAS Pt disk electrode as seen under 10X magnification before (A) and

+2 after (B) Fe(vbpy)3 film polymerization.

66 Although this electropolymerization is exciting, it is important to remember that in the spectroelectrochemical sensor, films are coated onto an optically transparent electrode, namely indium tin oxide (ITO). An attempt at this same process while replacing the BAS Pt disk electrode with an ITO slide (10 x 45 mm) is shown below

(Figure 4.7). Again, visual confirmation of a mechanically stable film is shown, as well

(Figure 4.8).

3.0x10-3 3.0x10-3

2.5x10-3 A -3 B 2.5x10

2.0x10-3 2.0x10-3

1.5x10-3 1.5x10-3

1.0x10-3 1.0x10-3 Current, A Current, A -4 5.0x10 -4 5.0x10

0.0 0.0

-4 -5.0x10 -5.0x10-4

-600 -800 -1000 -1200 -1400 -1600 -1800 1500 1000 500 0 -500 -1000 -1500 -2000 Potential, mV Potential, mV

+2 Figure 4.7. Electropolymerization of 3mM Fe(vbpy)3 in 0.1M tBAPF6/CH3CN vs.

quasi Ag/AgCl at 100 mV/s (A), and investigation of the resultant

+2 Fe(vbpy)3 film on an ITO slide working electrode in supporting

electrolyte only (B). Both cyclic voltammograms include a scan of a bare

ITO slide in supporting electrolyte only.

A B

Figure 4.8. Optically transparent ITO electrode as seen under 10X magnification

+2 before (A) and after (B) Fe(vbpy)3 film polymerization.

67 Initial electropolymerization shows far less defined vbpy redox peaks, and in several attempts using ITO, this was a typical result. Instead of the well defined 2 electron growth process shown on Pt, two initial peaks at -1.4 and -1.6 V increase into one composite peak at -1.65 V. In all attempts, the reverse wave on ITO, if present at all, is

+2 in the form of a composite wave found at -1.4 V. Despite this difficulty, a Fe(vbpy)3 still appears to form as confirmed in Figure 4.7B and Figure 4.8, and also appears to be mechanically stable. Electrochemically, the Fe+3/+2 peak at 1.0 V is accompanied by a pre-wave of sorts at +0.5. This secondary wave has not been previously reported and a smaller version of the anodic wave only may be present with the Pt electrode surface at the same potential in Figure 4.5B. The surface area of ITO is far greater than that of the

Pt disk (approximately 2.0 cm2 vs. 1.6 mm2), and along with magnifying peak currents overall, this pre-wave appears to be enhanced, as well, and may be due to another Fe redox couple or electrochemistry that may be occurring on an uncoated portion of the working electrode. Visually, the film is much less uniform across the ITO surface, with more cracking and less uniform surface coverage. This is thought to be due simply to the fact that a much larger surface area is being coated as compared to that of the small Pt disk electrode. Surface irregularities such as microscopic peaks and valleys present on the ITO slides may also play a role in the coating process.

In past studies, hybrid optically transparent electrodes composed of thin metal layers coated onto ITO slides have provided enhanced features over those of plain ITO, particularly for organic compounds.41 With ITO functioning as the working electrode in the spectroelectrochemical sensor, it was important to explore all varieties of ITO based

+2 transparent electrodes in the electropolymerization of Fe(vbpy)3 films. In accordance

68 with the procedure described previously, electropolymerization attempts were made on both Pt-ITO and Au-ITO hybrid electrodes, those electrodes remaining available following earlier investigations. Disappointingly, neither hybrid electrode showed particularly promising results in the electropolymerization as detailed for Pt-ITO and Au-

ITO in Figures 4.9 ,4.10, and 4.11.

5.0x10-3

5.0x10-3 A 4.0x10-3 B

4.0x10-3 3.0x10-3

-3 3.0x10 -3 2.0x10

-3 2.0x10 1.0x10-3 Current, A Current, Current, A Current,

-3 1.0x10 0.0

0.0 -1.0x10-3

-1.0x10-3 -2.0x10-3 -600 -800 -1000 -1200 -1400 -1600 -1800 1500 1000 500 0 -500 -1000 -1500 -2000 Potential, V Potential, mV

+2 Figure 4.9. Electropolymerization of 3mM Fe(vbpy)3 in 0.1M tBAPF6/CH3CN vs.

quasi Ag/AgCl reference at 100 mV/s (A), and investigation of the

+2 resultant Fe(vbpy)3 film on a Pt-ITO hybrid electrode in supporting

electrolyte only (B). Microscopy of the resultant film at 10x

magnification is also shown (C). Both cyclic voltammograms include a

scan of a bare Pt-ITO hybrid electrode in supporting electrolyte only.

69 -3 3.0x10 2.0x10-3

2.5x10-3 A 1.5x10-3 B

2.0x10-3 1.0x10-3 1.5x10-3 5.0x10-4 1.0x10-3 0.0 Current, A Current, Current, A 5.0x10-4

-5.0x10-4 0.0 bare

-3 -5.0x10-4 -1.0x10

-3 -1.0x10-3 -1.5x10

-600 -800 -1000 -1200 -1400 -1600 -1800 1500 1000 500 0 -500 -1000 -1500 -2000 Potential, mV Potential, mV

+2 Figure 4.10. Electropolymerization of 3mM Fe(vbpy)3 in 0.1M tBAPF6/CH3CN vs.

quasi Ag/AgCl at 100 mV/s (A), and investigation of the resultant

+2 Fe(vbpy)3 film on a Au-ITO hybrid electrode in supporting electrolyte

only (B). Both cyclic voltammograms include a scan of a bare Au-ITO

hybrid electrode in supporting electrolyte only.

A B

Figure 4.11. Optically transparent Pt-ITO hybrid electrode (A) and Au-ITO electrode

+2 (B) as seen under 10X magnification and coated with an Fe(vbpy)3 film.

The hybrid Pt-ITO electrode behaves very similarly to ITO, reflecting dominance of the ITO layer (~130 nm) with respect to a very thin Pt layer (75 nm) (Figure 4.9).

Vinylbipyridine peaks are ill-defined and only a small reverse, anodic wave is visible.

70 The Fe+3/+2 couple again appears at approximately +1.0 V, with a secondary peak at +0.5

V. The secondary peak reflects the Pt-ITO hybrid character as it is not as enhanced as on

ITO and has an asymmetric anodic peak, as was seen on Pt. The visual film formed is also a composite of sorts with a more uniform coverage typical of that on a Pt disk surface (Figure 4.11A). Hybrid Au-ITO performs even more poorly, with barely increasing vbpy peaks, which if it were not for visual film confirmation, would suggest little or no film formation (Figure 4.10 and 4.11B). Accordingly, the cyclic voltammogram in supporting electrolyte alone shows a possible vbpy cathodic wave at -

1.3 V, but also shows peaks characteristic of gold oxide formation at +1.0 and +0.5 V.

After review of electropolymerization on Pt disks, ITO slides, Pt-ITO and Au-

ITO hybrid slides, it is clear that the most reproducible and reliable electropolymerization

+2 for Fe(vbpy)3 films is found to be on the Pt disk working electrode. This is a difficult direction to take, particularly because Pt disk electrodes do not easily lend themselves to spectroscopic study. Transmission spectra of all optically transparent electrodes coated

+2 with a film of Fe(vbpy)3 were inconclusive due to light scattering and incompletely uniform surface coverage of the slide surface.

Due to success seen on the Pt disk electrode, it was decided that

+2 electropolymerization of the Zn(vbpy)3 complex would move forward using an

+2 identical procedure (Figure 4.12) which agrees with results seen in previous Zn(vbpy)3 work.92

71 5.0x10-5 0.000030 A 4.0x10-5 B 0.000025 3.0x10-5 0.000020 2.0x10-5 0.000015 1.0x10-5 0.000010

Current, A Current, 0.0 Current, A Current,

0.000005 -1.0x10-5

0.000000 -2.0x10-5

-0.000005 -5 -3.0x10

-0.000010 -4.0x10-5 -800 -900 -1000 -1100 -1200 -1300 -1400 -1500 1500 1000 500 0 -500 -1000 -1500 -2000 Potential, mV Potential, mV

+2 Figure 4.12. Electropolymerization of 3mM Zn(vbpy)3 in 0.1M tBAPF6/CH3CN vs.

quasi Ag/AgCl reference at 100 mV/s (A), and investigation of the

+2 resultant Zn(vbpy)3 film on a Pt disk electrode in supporting electrolyte

only (B). Both cyclic voltammograms include a scan of a bare Pt disk

electrode in supporting electrolyte only.

Electropolymerization (Figure 4.12A) proceeds with a uniform increase in vbpy wave growth; however, it is interesting that two separate peaks are not defined, reminiscent of

+2 the behavior of Fe(vbpy)3 film growth on ITO. The presence of the reverse wave and its uniform growth should be noted along with its asymmetry compared to the cathodic peak . Upon investigation of the film in a larger potential window, characteristic vbpy peaks appear at -1.5 V. Zn+2 is electrochemically inactive, therefore, no redox couple is present in the electrochemical window. Visual confirmation of the film is more difficult

+2 +2 as Zn(vbpy)3 in solution is virtually colorless, whereas Fe(vbpy)3 is a strong, nearly opaque red solution. At 10x magnification however, a change in the Pt disk surface is present again as a mottled, uniform film (Figure 4.14).

72 +2 Following Zn(vbpy)3 film formation and confirmation in supporting electrolyte, the film was immediately exposed to FeCl2 in 0.1M tBAPF6/CH3CN. (Although results

+2 presented here are for Fe in non-aqueous solvents, similar attempts with FeCl2 in water were made and achieved similar levels of success. This is important in light of potential spectroelectrochemical sensor use in aqueous solvents.) Reductive cycling from –1.5 to

+2.0 V in the FeCl2 solution was performed approximately 5-10 times and showed diminished vbpy-based waves and the appearance of an Fe+2 based peak at approximately

+1.0 V. These results might suggest that film structure is degrading and the appearance of the Fe+3/+2 redox couple on bare Pt is growing in were it not for visual confirmation of film presence (Figure 4.14). Electrochemistry of the initial exposure to FeCl2 and investigation of the exchanged film in supporting electrolyte only is shown in Figure

4.13A. The exchanged film was then placed in supporting electrolyte only and scanned through the full potential window revealing the appearance of an Fe+3/+2 peak at +1.0 V and significantly diminished vbpy-based reductive peaks (Figure 4.13B). It is significant that there has been no previous reporting of the electrochemistry of this exchange event.

It is, at best, poorly defined, but is consistent with suggestions in literature92 that several hours of repetitive cycling both in the FeCl2 solution and in supporting electrolyte following exchange is necessary for film reorganization and reappearance of characteristic Fe+3/+2 and vbpy-based waves. In this experiment, significant reappearance of waves at current magnitudes prior to the exchange was never seen, but extensive time-based cycling was not performed.

Removal of the Pt electrode from the Fe+2 containing solution reveals a film of

+2 reddish-orange hues similar to that seen with Fe(vbpy)3 polymerized alone (Figure

73 4.6). The exchanged Zn+2/Fe+2 film is mechanically stable and upon vigorous rinsing, neither film nor color dissipates. Figure 4.14 visually details the exchange using microscopy.

3.0x10-5 6.0x10-6 A B

2.0x10-5 bare Pt -6 Pt + Zn(vbpy) +2 film 4.0x10 3 Pt + Fe(vbpy) +2 film 3 1.0x10-5 2.0x10-6 Current, A Current, A

0.0 0.0

-6 bare -5 -2.0x10 -1.0x10

1500 1000 500 0 -500 -1000 -1500 -2000 2000 1500 1000 500 0 -500 -1000 -1500 -2000 Potential, mV Potential, mV

Figure 4.13. Exchange of Zn+2 for Fe+2 in vbpy based films detailed. (A) Shows 2

+2 examples from 5 cycles during exposure of Zn(vbpy)3 films to 10 mM

FeCl2 in 0.1 M tBAPF6/CH3CN vs. Ag/AgCl reference at 100 mV/s. (B)

+2 Shows the newly formed Fe(vbpy)3 film alongside bare Pt disk and

+2 previous Zn(vbpy)3 film. Both cyclic voltammograms include a scan of

a bare Pt disk electrode in supporting electrolyte only.

+2 Zn(vbpy)3 electropolymerizatio Exposure to FeCl2 n with electrochemical BAS bare Pt disk cycling

Figure 4.14. BAS Pt disk electrode as seen under 10X magnification detailing the

+2 +2 +2 formation of a Zn(vbpy)3 film and exchange of Zn with Fe to form a

+2 film of Fe(vbpy)3

74 Reflectance Measurements. Although visual and electrochemical data suggest

+2 +2 +2 the formation of a Zn(vbpy)3 film and exchange with Fe to form a Fe(vbpy)3 film, spectroscopic confirmation is necessary to provide the optical data upon which a spectroelectrochemical approach could be based. Unfortunately, experiments on ITO and other optically transparent electrodes are erratic and irreproducible, yielding films with surface coverage less uniform than those vbpy-based films formed on a standard BAS Pt disk electrode. Investigation of this non-transparent surface necessitated modification to an already exisiting set-up in order to obtain reflectance measurements of the films allegedly formed.17 According to absorbance and fluorescence spectra, it can be

+2 expected that a film of Zn(vbpy)3 would be fluorescent with emission centered around

340 nm. Upon exchange with Fe+2, this fluorescence would be quenched, as

+2 Fe(vbpy)3 (aq) shows no emission profile in the near-UV/visible range. Conversely,

+2 +2 Zn(vbpy)3 films are expected to be non-absorbing at 540 nm, whereas Fe(vbpy)3 (aq) shows a peak absorbance at this wavelength.

Based on the instrumental configuration described earlier, transmission data were collected yielding transmission (T) spectra which could be converted to absorbance (A) spectra (A=log Io/I or –log T). For fluorescence measurements, the light source was simply switched to a HeCd laser (λ=325 nm) with collection through the existing device yielding emission spectra. The instrumental design allowed the Pt disk electrode to be removed at will and measurements were recorded at the following intervals: bare Pt disk

+2 electrode prior to electropolymerization, following film formation of Zn(vbpy)3 , and

+2 +2 +2 following Zn /Fe exchange to form a film of Fe(vbpy)3 .

75 Raw transmission and calculated absorbance spectra are shown in Figure 4.15. In the raw transmission data, peaks clustered around 460 nm are due to the xenon arc light source. Absorbance spectra (Figure 4.15B) show the appearance of a broad based peak

+2 at around 540 nm, indicative of the presence of the Fe(vbpy)3 film following exchange.

This confirms what was suggested with electrochemical and visual data previously.

120000 Pt disk only 1.0 Zn(vbpy) +2 film B A 3 0.9 alleged Fe(vbpy) +2 film 100000 3 Zn(vbpy) +2 Film on Pt disk 0.8 3

80000 0.7

0.6 Fe(vbpy) +2 Film on Pt disk 3 60000 0.5 Raw Counts Absorbance, a.u. Absorbance, 40000 0.4

0.3 20000 0.2

0 0.1 300 350 400 450 500 550 600 650 700 250 300 350 400 450 500 550 600 650 700 Wavelength, nm Wavelength, nm

Figure 4.15. Transmission (A) and Absorbance (B) data collected during the exchange

+2 +2 +2 of Zn for Fe thereby forming Fe(vbpy)3 films on a Pt disk electrode.

Fluorescence data further confirm the Zn+2/Fe+2 exchange, with emission spectra shown in Figure 4.16. High intensity at 330 nm is due to scattered light from the 325 nm laser. A small peak around 440 nm is probably a plasma line from the HeCd laser.

76 40000

35000 Pt disk only Zn(vbpy) +2 film on Pt disk 30000 3 Fe(vbpy) +2 film on Pt disk 3 25000

20000

Raw Counts Raw 15000

10000

5000

0 320 340 360 380 400 420 440 460 Wavelength, nm

Figure 4.16. Fluorescence emission data collected during the exchange of Zn+2 for Fe+2

+2 thereby forming Fe(vbpy)3 films on a Pt disk electrode. Integration time

was 100 msec and laser power was ≤ 0.1 mW.

+2 It can be seen that Zn(vbpy)3 films have a broad profile typical of a solid-state emission

+2 and is slightly shifted from the Zn(vbpy)3 solution fluorescence (λem = 340 nm). The

+2 newly formed Fe(vbpy)3 film quenches any fluorescence to return to the baseline measurement of the Pt disk electrode alone, further confirming the Zn+2/Fe+2 exchange.

Conclusions

These proof-of-concept experiments provide the framework on which a metal-

- ligand complex based templating system could be tailored for use in the TcO4 spectroelectrochemical sensor; however, several factors are important to remember.

+2 +2 Chemical synthesis of both Fe(vbpy)3 and Zn(vbpy)3 resulted in compounds frequently including water as part of their structure. Some difficulties encountered

77 during electropolymerization, particularly on ITO, might possibly be less pronounced with a starting material that is dry. With extensive vacuum drying and no ability to apply heat, this proves to be a challenge. Also remember that the only approach taken here

+2 involved electropolymerization as a means of forming films of both Fe(vbpy)3 and

+2 Zn(vbpy)3 . Again, presence of water and/or oxygen potentially play a huge role in detracting from easy film formation. While success was seen, it was not simple or straightforward. Carrying out these experiments in a glove box could prove to be useful, but physically too difficult, particularly for long-term use as a sensor component. An approach using chemical initiation of vbpy-based polymerization and subsequent chemical linkage to an optically transparent surface appears to be a more reliable and reproducible route to a film template for Tc-ligand complexation in the future. Vinyl group dependent chemical polymerization is quite common and offers with it, well understood chemical processes.

There are advantages and disadvantages to both strategies. Electropolymerization carries with it some procedural difficulty, but offers a “one-pot” sensor, wherein a film can be generated in the same instrumental compartment where sample introduction and detection take place. Attention to possible interaction between all of these components may be necessary, but overall electrochemical control could be quite useful. Chemical synthesis and attachment to an ITO surface have the potential for a much more uniform

(both in physical structure and surface coverage) film; however, this approach brings with it all current film use issues including stability, film dynamics, and the additional task of manufacturing films on surfaces outside of the sensor compartment.

78 What is most promising though is the possibility of a successful metal-ligand couple for use in the spectroelectrochemical sensor. There is potential for monitoring with both absorbance and/or fluorescence and these steps represent the natural direction for further development of the sensor.

79 Chapter 5.

An Instructional Laboratory for Making and Using a Material to Sense

Cu+2.

Introduction

In recent years, the term chemical sensor has become quite common. We encounter chemical sensors on an everyday basis in the form of carbon monoxide detectors, blood glucose monitoring devices, automotive oxygen sensors, the

BreathalyzerTM blood alcohol content analyzer, and many others. These devices convert chemical information into a measured signal to quantify a specific analyte.

The field of chemical sensors is growing at a rapid rate with an approximately

10% annual increase in its reference database of more than 20,000 articles.94 Of note, roughly 40% of these references are found in journals with some focus on chemical sensors, but the remainder are scattered throughout the literature, indicative of a broad based interest in the topic.95 The past 20 years have been filled with amazing developments in computer, miniaturization, and new materials technologies. Chemical sensors have been developed largely because they satisfy the need for a compact device that can acquire large amounts of data and convert it to user-friendly output, all of which is facilitated by and is in conjunction with these technological advances.

New materials technology includes continued development of polymers, the uses of which are commonplace, making their importance in our everyday lives immense.

Life itself depends upon polymers of nucleotides, amino acids, and sugars for its very existence. Plastics are polymeric materials used in everything from automobiles to

80 computers. Polymers can have ecological impact, as well, with a NASA-developed material patented by Philipp, et al. used in environmental cleanup, removing heavy metals from water or aqueous systems.96-99

With both polymers and chemical sensors having become such an integral component of both everyday life and wide-ranging scientific research, it is important that introduction to both topics occur early in the academic experience, serving students with an experience in two broad-based fields, supplying educated researchers as well as sustaining interest in both fields. A well-defined sensor is typically anchored in the basic principles of chemistry, biology, and physics, easily lending itself to the academic forum.

There may be some introduction to sensors in a lecture-based chemistry course (e.g., instrumental analysis), but there is a definite lack of simple, hands-on laboratory experiences, as evidenced by the availability of few innovative sensors related experiments, a fraction of which are geared towards education.100-102 Polymer chemistry enjoys a more widely accepted reputation in academia with a Journal of Chemical

Education Project Chemlab search of "polymers" yielding 100+ results. However, utilizing polymeric materials as a component of a chemical sensor yielded no results.

Data presented here can be used to fill that gap, providing the basis for a simple chemical sensors related experiment rooted in the synthesis of polymeric materials for use in either an advanced high school or undergraduate collegiate laboratory. Students would be introduced to and combine the concepts of the chemical sensor, polymer chemistry, spectroscopy, metal chelates, and quantitative analytical methods.

A porous, cross-linked polymer network doped with a spectroscopically active chelating agent can act together as a component of a chemical sensor. Poly(vinyl

81 alcohol), PVA, may be cross-linked with glutaraldehyde in the presence of the catalyst

+ H3O . A polyelectrolyte (poly(acrylic acid), PAA, acts as an ion-exchange medium for cations and is entrapped in the network (Figure 5.1), which functions as a semipermeable hydrogel.

PVA

H2 H2 H2 H2 C C C C CH CH CH

OH OH OH H2 H2 H2 H2 C C C C CH CH CH O H C OOOH + CH CH2 H3O

H2C CH2

CH2 CH2 glutaraldehyde glutaraldehyde CH 2 cross-link C CH H O O O HO OH OH OH CH CH CH C C C C CH CH CH H H2 H H2 2 C C C C 2 H H H 2 H2 2 2

H 2 H 2 H 2 H 2 C C C C PAA CH CH CH

COOH COOH COOH

Figure 5.1. Crosslinking of PVA with glutaraldehyde. Network entrapped PAA

(protonated form) is shown at bottom.

The chelating agent 1-(2-pyridylazo)-2-naphthol, PAN, can be incorporated into this network, adding a binding site or selective element for a +2 metal ion to the matrix.

+2 Copper sulfate (CuSO4) solutions can serve as a source of Cu ions that in turn interact

82 with PAN to yield a PAN-Cu complex. By exposing the PVA-PAA/PAN disk to a series of aqueous Cu+2 solutions of varying concentration, after a fixed exposure time, the relationship between the amount of PAN-Cu complex formed and Cu+2 solution concentration may be observed and quantified spectroscopically. A calibration plot can be generated from this data, and potentially used to identify concentration of an unknown

Cu+2 solution.

Experimental

Materials and Instrumentation. Poly(vinyl alcohol) or PVA (Aldrich), poly(acrylic acid) or PAA (Polysciences, Inc.), anhydrous cupric sulfate (JT Baker), glutaraldehyde (Acros), and hydrocholoric acid (Fisher Scientific) were all diluted or solvated to appropriate concentrations in deionized water obtained from a Barnstead water system (Dubuque, IA). 1-(2-pyridylazo)-2-naphthol or PAN was obtained from

Aldrich and was solvated in 50/50 deionized water/ethanol (Aaper Alcohol). 100 x 15 mm petri dishes, plastic cuvettes, and 28 x 57 mm glass vials (all disposable) were obtained from Fisher Scientific. Absorbance spectra were obtained using a Hewlett

Packard spectrophotometer. Data analysis was completed using Microsoft Excel.

Network Preparation. The following ingredients were mixed with a glass stir rod in a large disposable petri dish until homogeneous (ingredient order is specific):

15.0 mL 10% PVA

15.0 mL 10% PAA

1.50 mL 5% glutaraldehyde

1.50 mL 0.5 M HCl

5.44 mL 2x10-3 M PAN solution

83 The Petri dish was then covered and the disk allowed to cure for 7 days. On day 7, the disk was drained of excess liquid and allowed to dry uncovered (If the disk stuck to the edges of the Petri dish, its sides were loosened with a razor blade). The disk then cured to a transparent solid (approximately 5 days).

Sensing Cu+2. One day prior to experimentation, dried disks were pre- equilibrated in deionized water. The hydrated disk was then cut with a razor blade into pieces, each measuring approximately 1 cm x 2 cm. Disposable vials were filled with sufficient liquid volume to cover the polymer film sections (all vials should hold an equivalent volume of solution), with a section of the hydrated disk then placed in each vial and allowed to soak for 1 hour. After soaking, each film section was removed from its solution, placed in a disposable cuvette (flush against one side), and its absorbance spectrum recorded.

Results and Discussion

PAN-Cu+2 interaction. Aqueous solutions of PAN and Cu+2 interact, yielding a

1:1 PAN-Cu complex (λmax = 555 nm). Figure 5.2 spectroscopically illustrates this chelating event. It has been reported that PAN and Cu+2 can form a stable complex in a

2:1 ratio103, but we did not observe its formation.104 Absorbance of PAN contributes

4 -1 -1 105 greatly below 500 nm (ε465 = 1.8 x 10 M cm ) , but does not interfere with the

+2 -1 - absorbance maximum for PAN-Cu found at 555 nm. In excess is Cu (ε810 = 12 M cm

1)106 with its peak at 810 nm remaining relatively unchanged, thereby showing an insignificant amount of bulk Cu+2 being used to form the PAN-Cu complex.

84 2.5

PAN CuSO 2.0 4 PAN + CuSO 4 N 1.5 N N M 2+

Absorbance 1.0 O H

0.5

0.0 400 500 600 700 800 900 1000 Wavelength/nm

-5 Figure 5.2. Absorbance spectra of 8x10 M PAN, 0.05M CuSO4, and resultant PAN-

Cu complex. PAN-Metal+2 complex is shown at right.

Network synthesis and Cu+2 exposure. Synthesis of the PVA-PAA/PAN doped network is straightforward with hydrogel curing occurring over several days time during which the thick, monolithic gel is drained of excess water. Equilibration of this dried polymer network in water is necessary prior to Cu+2 solution exposure in order to avoid any effect that network swelling may have on the uptake of Cu+2 into the matrix. This uptake is time dependent with longer exposure time resulting in greater preconcentration of ions into the matrix. An upper limit is reached around 3 hours of exposure, with spectra generated from these network sections representing a Cu+2 concentration in the film section that is too high to accurately determine spectrophotometrically. It is recommended that spectra be obtained approximately 1 hour following initial network section exposure to CuSO4 solutions whose concentrations were chosen to fall between

85 0.025 to 0.20 mM, well within the linear range of the PAN-Cu relationship. A background distilled water spectrum was subtracted from raw spectral data and resultant profiles were normalized, giving the final results shown in Figure 5.3. Noise below 500 nm is attributed to highly absorbent PAN and anomalous behavior at 651 nm is an artifact of the Hewlett-Packard spectrophotometer. Matrix effects do not seem to affect PAN-Cu complex formation. The spectrum profile remains unchanged upon chelation in the

107 polymer network versus in solution with λmax at 555 nm in both cases.

2.0

(e) 1.5

(d)

1.0

Absorbance/au (c)

0.5

(b) (a) 0.0 400 450 500 550 600 650 700 Wavelength/nm

Figure 5.3. Absorbance spectra of PVA-PAA/PAN network sections soaked for 1

hour in CuSO4 solution at a concentration of (a) 0.025 mM, (b) 0.05 mM,

+2 Increased Cu concentration present in CuSO4 solutions results in increased concentration of PAN-Cu complex. At λmax = 555 nm an absorbance maximum corresponding to each concentration may be obtained and a calibration plot of absorbance vs. CuSO4 solution concentration was generated (Figure 5.4).

1.8

1.6

1.4

1.2

1.0

0.8

Absorbance/au 0.6

0.4

0.2

0.0 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 Concentration/mM

Figure 5.4. Calibration plot for the PAN-Cu complex at λmax=555 nm (from spectra

shown in Figure 5.3).

Concentrations of unknown Cu+2 solutions can be identified using this plot.

Based on this linear relationship, PVA-PAA/PAN doped networks would be a useful component of a chemical sensor geared towards +2 metal ions such as Cu+2.

Conclusions

This chapter represents the nuts and bolts of a successful materials/chemical sensors related laboratory experiment which is to be published later.108 Results are quite reproducible and the experiment was generally popular with students. They enjoyed the start-to-finish nature of the project, where a material they made was then used in a potentially real-life application. Not including the cost of the spectrometer, materials for a typical class (chemicals, disposable cuvettes, etc.) are expected to cost approximately

87 $500.00. This quantity of material is sufficient to complete a laboratory (using 30 students) about 25 times. The only items requiring regular restocking after initial start-up include PVA, PAA, glutaraldehyde and disposable cuvettes. For many schools and colleges/universities this skeleton can provide the basis for extensive investigations into polymers and materials chemistry as they both apply to chemical sensing.

88 References Cited

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97 Appendix I.

Determining Diffusion Coefficients of Analytes in a Thin Film as Used in the Spectroelectrochemical Sensor: A Step-by-Step Guide.

98 PART ONE: EXPERIMENTAL

For Each Slide - (Remember to complete the following on Bare ITO first)

(1) Soak 2x films 16-24 hours prior to experiment in supporting electrolyte. One is

for the following protocol and the other is its blank for use in step (5)

Use IR compensation mode, remembering to test the slide in use and enable IR

compensation for all runs following.

(2) Monitor uptake in analyte using BAS-100 sweep technique: cyclic voltammogram

Continue constant monitoring, plotting CV peak current (remember to use the

correct segment) vs. time, until saturation is achieved.

(3) Once saturation occurs, vary scan rate of CV (remember to check sensitivity/gain

setting, and keep all other settings the same), using at least 7 different scan rates

spanning the window available on the BAS (1-51200 mV/s). When finished,

check integrity of the system by running a CV using original scan rate and

conditions from (2). If CV’s agree, continue.

(4) Use chronoamperometry to generate a plot of current vs. time. This part is tricky!

You must know what you are looking for (i.e. a step showing charging current, a

diffusion controlled region, transition area, and return to equilibrium). One

approach is to vary the pulse width, spanning the entire window available (BAS-

99 100 offers pulse widths from 1 to 32000 msec). You must have a quiet time of

120 seconds or more, to make sure the system is stable.

(5) When finished, remove the slide from the solution and take its spectrum vs. that

of a slide soaking for the same total amount of time in supporting electrolyte.

Determine absorbance of analyte in the film. An alternate method is to take a

spectrum of the solution before and after saturation with the difference being the

absorbance in the film (accounting for film volume vs. solution volume)

(NOTE: Your analyte must be used in a colored form for this absorbance

determination)

You are now ready to begin diffusion coefficient calculation.

PART TWO: CALCULATIONS

(6) While monitoring saturation in (2), plot CV peak current vs. time (remember to

use the appropriate segment depending on your redox system). Estimate a

loading time for the film from this plot.

(7) Calculate absorbance in the film. Be sure to have units of mol/cm3 (a.k.a.

mol/mL).

100 (Remember that pathlength may be the thickness of the film. If measuring

solutions, not films, you must convert the difference in absorbance to moles and

then place that number of moles in the volume of the film.)

(8) Choose the chronoamperogram used to calculate D. Make sure current is in

Amps and time is in seconds. Separate forward and reverse step, plotting only

that step which is specific to your analyte diffusing into the film. Demarcate

charging current, diffusion controlled region, transitional area, and equilibrium on

this plot.

(9) From (8) create a Cottrell Plot (current vs. 1/t1/2). Demarcate the same regions

from (8) on this plot, noting that charging current begins at the largest time value.

(10) Select ONLY the diffusion controlled region from the Cottrell Plot in (9) and plot

this alone. This slope will be used to calculate D (in cm2/s), using the Cottrell

equation.

(11) Create a Randles-Sevcik plot of peak current vs. scan rate1/2 (Use data from (3)).

Use the slope of this line to calculate D (in cm2/s) and compare it to the value

obtained in (10).

101 (12) From both D values, calculate a loading time for the film using the Einstein

equation. The value obtained should be similar to that observed experimentally

(6), or at least in the same order of magnitude.

102 Appendix II.

Instructional Laboratories as used in Freshman Honors Chemistry,

University of Cincinnati, Ohio.

Acknowledgments:

This is a compilation of laboratories as used in Freshman Honors Chemistry at the

University of Cincinnati from 1997-2002, and as such represents the work of several individuals: Dr. Carl J. Seliskar, Dr. Estel Sprague, and Dr. Thomas H. Ridgway (all from the University of Cincinnati); Dr. Letian Gao (Flint Ink; Detroit, MI) ; and the scientists at NASA-Glenn Research Center, particularly Dr. Kenneth W. Street and his group.

103 DATA ANALYSIS

At the heart of experimental science is data gathering and analysis. Intimately associated with the gathering of experimental data is the occurrence of errors as a part of measurement. Indeed, an incorrect error assessment has sometimes led to the premature death of claimed scientific breakthroughs. Perhaps a good example of this is the “cold fusion” fiasco of recent years. It is therefore appropriate that we turn to an introduction of experimental errors and the influence they have on interpretation of scientific “facts.”

First, we need a delineation of some issues involved along with a few definitions. It almost seems to be part of our genetic composition to hold that if we do something often enough we will do it better. Another way to say this is to use the trite expression “practice makes perfect”. In either of these tales there is an implicit assumption that in doing some action (experimental measurement), repetition will yield a “better” outcome (one closer to the true value). So it is in experimental science where we make the assumption that repetition in measurements will lead us closer to the true value of some quantity. The determination of Avogadro’s number is a good example - each more sophisticated new measurement is assumed to be better than previously held values.

104 There are two types of common (legitimate) experimental errors: systematic error and random error. So where do stupid mistakes in calculation or measurement fall? Such mistakes are thought of as “illegitimate errors” and not given the pedigree of legitimacy.

This is because they are easily recognizable and remedied. On the other hand, legitimate errors persist and certainly can influence our determination of estimates of the “true” values of quantities.

Systematic errors are not easy to recognize because they occur reproducibly from measurement to measurement. They can be the total undoing of the value of a measurement. For example, you might precisely and repeatedly determine the length of your pet snake to the nearest one-tenth of a millimeter using a ruler without ever knowing that your little brother sawed the first 2.3 millimeters off the other end of it! Of course, no amount of averaging of your measurements will lead to a better value of the snake’s true length. Your best value would still be at least 2.3 millimeters short of the true value. At best, systematic errors can be estimated from surrounding experimental conditions at the time of measurement.

Accuracy and precision are two very different things! Accuracy relates to the closeness of a measurement to the true value. Precision is a measure of how exactly the measurement is made. Precision is also a measure of how reproducible a measurement is. The term relative precision indicates the uncertainty in terms of a fraction of the value of the result. In the case of the length of your pet snake, you may have precisely

105 determined the snake’s length by repeated measurement, but without adjustment for your little brother’s deed you have not determined the snake’s length accurately.

Significant digits (figures) and the rounding of numbers are not difficult concepts to master. In a number the significant digits are determined by:

1. The leftmost nonzero digit is the most significant digit.

2. If there is no decimal point, the rightmost nonzero digit is the least

significant digit.

3. If there is a decimal point, the rightmost digit is the least

significant digit, even if it is a 0.

4. All digits between the least and most significant digits are

significant digits.

In experimental science it is traditional to give one more digit for an observation than the actual number of significant digits. When insignificant digits are dropped from a number, the last digit retained should be rounded off for best accuracy. To round off a number to a smaller number of significant digits than originally given, truncate the number to the desired number of significant digits and treat the excess as a decimal fraction:

1. If the fraction is greater than 1/2, increment the least significant

digit.

2. If the fraction is less than 1/2, do not increment.

106 3. If the fraction is 1/2, increment the least significant digit only if it

is odd.

Consider the following example number: 1.23564 This number, as written has 6 significant digits, the leftmost (1) being the most significant, the rightmost (4) being the least significant. If we wish to represent this number with fewer significant digits, say 4 digits, the result would be 1.236. If we preferred 5 significant digits, the resulting rounded number would be 1.2356. If we preferred 3 significant digits, the number would be 1.24.

Now let’s turn to random errors. Accuracy is mostly determined by our ability to control systematic errors, while precision is controlled by how well we control random errors. Random errors, as the name suggests, are distributed randomly (often normally and/or symmetrically) and if we make enough experimental determinations of a number these random errors tend to “average out” to yield an average value, which is a good estimate of the true value providing there are no systematic errors.

So how do we do this averaging stuff? Let’s learn this using an example laboratory experiment where we determine the length of your pet snake to the nearest millimeter using a good ruler. Suppose we make 100 determinations of this length, X, and that some of these numbers are the same and, thus, have a frequency of occurrence , f, which is larger than 1. This means that for some possible values of the length we get an f of zero;

107 for others f may be a much larger integer. Below is a table which represents a series of length measurements:

X, cm f fX X!Xave (X!Xave)**2 f(X!Xave)**2 18.9 1 18.9 -1.128 1.2616 1.262 19.0 0 0 -1.028 1.0568 0.0 19.1 1 19.1 -0.928 0.8612 0.861 19.2 2 38.4 -0.828 0.6856 1.371 19.3 1 19.3 -0.728 0.5300 0.530 19.4 4 77.6 -0.628 0.3944 1.578 19.5 3 58.5 -0.528 0.2788 0.836 19.6 9 176.4 -0.428 0.1832 1.649 19.7 8 157.6 -0.328 0.1076 0.861 19.8 11 217.8 -0.228 0.0520 0.572 19.9 9 179.1 -0.128 0.0164 0.147 20.0 5 100.0 -0.028 0.0008 0.004 20.1 7 140.7 0.072 0.0052 0.036 20.2 8 161.6 0.172 0.0296 0.237 20.3 9 182.7 0.272 0.0740 0.666 20.4 6 122.4 0.372 0.1384 0.830 20.5 3 61.5 0.472 0.2228 0.668 20.6 2 41.2 0.572 0.3272 0.754 20.7 2 41.4 0.672 0.4516 0.903 20.8 2 41.6 0.772 0.5960 1.192 20.9 2 41.8 0.872 0.7604 1.521 21.0 4 84.0 0.972 0.9488 3.775 21.1 0 0 1.072 1.1492 0.000 21.2 1 21.2 1.172 1.3736 1.374

SUM 100 2002.8 22.627

In this table we have also listed quantities derived from f and X as explained below. Let us further define two quantities derived from the data set, namely, the sample mean value and the sample variance:

108

Xave = 2002.8/100 = 20.028 cm Sample Mean Value (“the average”)

s2 = 22.627/(100-1) = 0.229 cm2 Sample Variance

(from which the square root gives)

s = 0.48 cm Sample Standard Deviation

In general, these quantities are defined as

Sample Mean, Xave:

Xave = {1/N} ∑ Xi i Sample Variance, s2:

2 2 s = {1/(N - 1)} ∑ (Xi - Xave) i

where the summation index “i” ranges over all observations N (in the case above we made 100 measurements so N = 100).

Sample Standard Deviation, s

s = √s2

Usually one reports the sample mean value and the standard deviation together as

Xave ± 1 s or Xave ± 2 s .

The reasons for reporting the measurement mean value with one or two sample standard deviations is a convention based on a more complete error analysis and one which is beyond our interests at this time. Suffice it to say that the standard deviation expresses the

109 confidence with which we have made the measurement expressed by the sample mean value. In the specific example of the snake’s length this result would be reported as

Xave = 20.03 ± 0.48 cm (1 s) or Xave = 20.03 ± 0.96 cm (2 s).

Either way you have a 20 cm snake on your hands!

Had we measured the snake many more times the distribution of errors would become more and more symmetrical about the mean value of the length. So we imagine that given enough stamina (by both the experimenter and the snake) we could refine our snake length until it approached the true value. In the limit of an infinite number of measurements the distribution of errors, namely, the frequency versus the value of X -

Xave, would become continuous and symmetrical about the mean value. In fact we can define a distribution of random errors as a normal distribution or Gaussian distribution using quantities which we have found experimentally:

2 1/2 2 2 P(X, Xave, s) = {1/(2πs ) } e^[-(1/2s )(X - Xave) ]

Such a nice function is symmetrically distributed about the sample mean value, has a half-width of 2.354s and is easily scaled to our actual data. The first term in the equation is the scaling factor and may be replaced with the maximum frequency recorded. This is shown in the graph below where both the sample data and the derived sample distribution are presented.

110 Student Exercise

One dark and boring January night, 199 first year chemistry students (Chem 10X, of course) banded together and decided to measure Avogadro’s number by a wide variety of experiments. So they all went about their own experiments (except for about 18 minutes when they were wolfing down cheese coneys) each trying to get the best value. Later that night they gathered to compare their results. In looking at each others results they noticed that there was a wide distribution of measured values even though each student had tried to make the best measurement possible. So in frustration they grouped together all the measured values of Avogadro’s number, rounded them to the nearest 0.005 x 1023 per mole, and summarized their results in the table shown below.

Frequency Avogadro’s Number N x 10**23 3 5.990 5 5.995 9 6.000 14 6.005 19 6.010 25 6.015 27 6.020 26 6.025 24 6.030 19 6.035 13 6.040 8 6.045 5 6.050 2 6.055

Since you are taking Honors Chemistry, they came to you to help them understand their results.

111 Using a computer spreadsheet program and the definitions introduced previously in this exercise, compute the following from their data:

1. The sample mean value

2. The frequency-weighted square of the deviations

3. The sample variance

4. The sample standard deviation

Using the graphics capabilities of the spreadsheet program, plot the following:

1. Frequency versus the measured value of Avogadro’s number

2. On the plot in part #1, also plot the normal distribution of errors for your

computed values of the sample mean and the sample standard deviation.

Be sure to write up your results in your notebook and make a diskette copy of your work.

* This laboratory is very much indebted to the late Philip R. Bevington of Case Institute of Technology and his “Data Reduction and Error Analysis for the Physical Sciences” for showing a way (albeit without snakes) to the rough analysis of experimental data without having to enter the magic kingdom of statistical mumbo jumbo.

112 MORE DATA ANALYSIS

(On Making Graphs and Stuff Like That)

Often we have the need to summarize the results of experiments where a certain outcome

(dependent variable) was measured with respect to an independently arranged parameter

(independent variable). An example would be the results of measurements of the volume of a gas as a function of the applied pressure at fixed temperature and mass of gas

(Boyle’s Law Revisited). We will look at this typical situation in two different ways. The first way (Part A) will be to represent the data “by hand” with a nice simple graph. The second way (Part B) will involve getting introduced to computer spreadsheet software for graphing and, in some cases, analyzing such data. Over time, the latter way will become familiar to you for the analyses of data sets generated in the laboratory.

First, let’s define a sample data set. We will take five representative readings from a

Boyle’s Law Revisited experiment done some time ago. In this experiment we measured the mercury column heights for different values of the leveling bulb containing liquid mercury. Having also recorded the prevailing barometric pressure in mm of mercury

(737.7 mm on the experiment day) and the value of the left-hand column top (99.22 cm), we then went about representing these measured values in tabular form:

O(i), cm C(i), cm 99.22 - C(i), cm O(i) - C(I), cm 57.40 52.21 47.01 5.19 64.00 54.55 44.67 9.45 74.68 58.02 41.20 16.66 88.09 61.87 37.35 26.22 98.63 64.54 34.68 34.09

113 In the table we also listed the derived quantities related to the pressure difference and the volume (see Boyle’s Law Revisited experiment for details).

Next, some general guidelines about 2-dimensional graphs:

1. It is tradition to plot a 2-dimensional graph by arranging the dependent variable on the ordinate (vertical axis) and the independent variable on the abscissa (horizontal axis). Thus, we might say that we plot the ordinate variable versus the abscissa variable.

An example would be to plot Y versus X for the well known linear relationship Y = mX

+ b.

2. In deciding the proportions for the graph, several factors are to be considered.

Choose an overall graph size that fairly represents the degree of precision which the data represents. In the example data set, each mercury column height was measured to four significant digits; to represent this relatively high precision we would choose to fill much of a notebook size page of graph paper (8.5” x 11”) to plot the data. If we had recorded the values to only 2 significant digits, we might have chosen a graph of smaller size.

While one can’t be dogmatic about this, when manually plotting use your best judgment about what the data precision requires given the finite size of graph paper.

3. In deciding the scales of each axis, choose divisions that represent easily understood values for interpolation between the data points. Usually this means that you would choose divisions that are separated by nice round numbers. (Excel usually does

114 this automatically for you but sometimes you have to change the scales chosen for you by the program.)

4. In completing your graph be sure to label the ordinate and the abscissa with the variables they represent and the chosen scales.

After choosing the graphical arrangement which best fits your data set, plot the data points as accurately as the chosen scale permits interpolating between scale divisions as necessary. If you wish you can highlight each data point by drawing a circle or similar figure around each plotted point. In Excel you must choose these options.

“Drawing lines” between the points on the graph is not always straightforward and to a large extent depends on the specific data set in question. If it known a priori that the relationship which the data set follows is linear, then a straight line can be constructed to represent the overall data. As a start you might “eyeball” this line as a rough estimate of the linear relationship. To construct the best straight line is a matter involving the statistical analysis of the data (linear regression) which, in its details, is beyond the scope of this course. Nonetheless, when such an analysis is appropriate, we will use Excel’s ability to accomplish this and give us the results of such an analysis.

Part A: Graphing Results by Hand

Graphing data “by hand” is an age-old technique that is still often used as the data is being acquired in the laboratory. Indeed, many laboratory notebook pages still show such

115 important first attempts at understanding data sets. Often a first graphical analysis can show the overall quality of a data set and the absence or presence of bad data points.

Even in the age of personal computers, graphical analysis by hand is an important tool in the experimentalist’s arsenal of weapons.

Let’s now plot the Boyle’s Law data set trying to follow the guidelines set out above. The first step in this analysis is to rationalize the measured quantities, O(i) and C(i), in terms of pressure and volume. The total pressure exerted on the gas is the sum of the prevailing barometric pressure (737.7 Torr) and the difference in column heights, O(i) - C(i) in millimeters of mercury. The volume of the gas is directly proportional to the difference between the top of the left-hand column (99.22 cm) and the recorded value, C(i). This leads us to the construction of a table of data points (V, P) to be plotted in a graph of V versus P:

“Volume”, cm Pressure, Torr 47.01 789.6 44.67 832.2 41.20 904.3 37.35 999.9 34.68 1078.6

From this table we see that the pressure (independent variable) varies from 789.6 Torr to

1078.6 Torr. So we might choose the range for the X-axis (P) as 750.00 to 1100.00 Torr.

For the Y-axis we see that, since the volume parameter varies from 47.01 to 34.68 cm, choosing the scale 30.00 to 50.00 cm would span the data variation nicely. You could also convert these “volume” numbers to the units of millimeters if you wished but we

116 will just keep the values in the units of cm. With these ranges in mind look at the graph on the following page where the data has been plotted. The X-axis (Pressure) was chosen so that 350 Torr variation was spread out over 35 divisions of quadrille paper or more exactly 10 Torr per 1/4 inch starting at 780 Torr. The Y-axis (Volume Parameter) was chosen so that 20 cm of “volume” variation was spread out over 20 divisions. In both cases the quadrille grid lines correspond to nice even numbers, namely, the X-axis: 10

Torr/division and Y-axis: 1 cm/division. This makes interpolation and plotting of the experimental points easy for either axis.

Now construct your own graph of V versus P using the 5 representative data points that you selected from the Boyle’s Law Revisited Experiment. Do this in your notebook as part of the analysis of the data you obtained.

Now that you have the 5 data points plotted, let’s turn to describing the relationship that the points represent. What does the ideal gas equation predict this relationship should be?

How would you indicate that on your graph? Can you think of a way to scale the ideal gas equation to your data points? (Hint: Consider the ideal gas equation PV = nRT. What values would n, R, and T have to have to yield your observed data points?)

Part B: Excel Computer Spreadsheet and Full Data Analyses

Your instructors will demonstrate the use of the personal computer and an example spreadsheet program. You will then learn to use this powerful tool, first to reproduce your

117 graphical analysis of the 5 data points selected from the Boyle’s Law Revisited experiment and, second, to analyze the full data set you acquired. The write-up for this experiment specifies a series of plots that you are to make using the full data set and the computer. Be sure to insert all of the hardcopy of your work (copy of the spreadsheet and all plots) in your three-ring binder.

118

119 BOYLE’S LAW REVISITED

ROBERT BOYLE (1627-1691)

I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind.

-Lord Kelvin (William Thomson, 1st Baron)

(1824-1907) English physicist and

mathematician. In: Popular Lectures and

Addresses, London, 1889, v. I, p. 73.

120 INTRODUCTION

The purpose of this experiment is to familiarize you with the equation of state of an ideal gas through an experimental examination of Boyle’s Law. This exercise also has an important secondary aim, that is, to get you to sharpen up your skills at acquiring, analyzing and documenting data, as well as at constructing quantitative graphs. The reason for this is very practical – often, the experimentalist first views and interprets his/her results by graphical analysis.

In 1662 Robert Boyle deduced the famous relationship between the pressure exerted on a fixed mass of gas at constant temperature and the volume of the gas. His observations can be stated several ways:

V α 1/P (1)

V = k[1/P] (2)

PV= k (3)

where k is a constant at fixed temperature. Equation (1) emphasizes that volume is inversely proportional to pressure; equation (2) that a constant of proportionality exists at fixed temperature which relates the inverse of the pressure to the volume; and, finally, equation (3) that the algebraic product of pressure and volume represents a constant

121 quantity, namely, k. Equation (3) also leads immediately to a useful relationship for a fixed mass of gas at fixed temperature:

P1V1 = P2V2 (constant n,T) (4) or

V2 = V1 [P1/P2] (constant n,T) (5)

Equation (5) is most useful in this experiment. Simply put, from the initial volume, V1, you can calculate a new volume, V2, by multiplication by the ratio P1/P2. (You might note that the ratio of the pressures does not depend on the units of pressure chosen.) In this experiment, pressure-volume data are taken on a sample of gas (air) at varying pressures. From this data set you are to construct several plots: volume versus pressure, volume versus reciprocal pressure and, pressure times volume versus pressure.

111111111111111111

CAUTION! In this experiment you will be using liquid mercury metal which is toxic. If a mercury spill occurs do not attempt to clean it up yourself but immediately inform the laboratory assistant who will treat the spill. Avoid breathing mercury vapor (the liquid metal has a very small vapor pressure at room temperature). You must wear safety glasses at all times while in the laboratory. Be careful with the Boyle’s law apparatus! It is expensive and not easily replaced.

122 The drawing of the apparatus which you will be using is shown (thanks to Prof. Sprague) on the last page of this handout. Looking at this drawing and, eventually, at your own equipment in the laboratory, you can see that it consists of a glass U-tube (one arm sealed, the other open to the air) and a leveling bulb, also open to the air. These two fixtures are joined through a plastic tube at the bottom of the U-tube at a “Y” connection.

The system has been charged with liquid mercury.

Getting Ready Before actually recording any values of the height of the mercury columns or volume of the entrapped gas, let’s think about the apparatus and the way the liquid mercury positions itself in the arms of the U-tube. You should answer the following questions in your notebook before you enter the lab.

1. Pressure is force divided by area, P = F/A, yet the experiment will require that you measure the heights of liquid mercury columns. What is the relationship between height and pressure? Between height and volume? Explain.

2. What are the forces and, therefore, the pressures at points “C” and “O” in the apparatus? What is the relationship of the height of the mercury at position “O” and the level in the leveling bulb?

3. Your laboratory instructor will determine the atmospheric pressure (while you watch) using the barometer on the side wall of the laboratory. How will this value affect your determination of the pressure on the mass of gas entrapped in the apparatus?

123

4. Do you need to know the actual mass of the gas (air) entrapped within the apparatus to verify Boyle’s law? Whether or not you need to know this quantity, can you think of a way to calculate it from a measurement you can make using the apparatus?

5. How precisely can you determine the heights of the mercury columns by using the scale (in centimeters or millimeters) on the apparatus? What determines how accurate these readings are?

PROCEDURE

The procedure for recording your results is as follows:

1. Record the number of the Boyle’s Law apparatus which you will use for this experiment. (You need to do this because each piece of equipment is unique.) SLOWLY raise the leveling bulb until the mercury column rises into the Y-connector and to a level which allows you to make the first mercury column height readings on the vertical scale.

Record the mercury levels in the two arms of the U-tube by determining the heights in millimeters. You should call these values Ci and Oi (for Closed tube initial level and

Open tube initial level). Also record the height of the top of the closed-column (Point E) on the U-tube.

124 2. Now construct a data table in your notebook to record the two measured column heights ( C1, ....., Cn and O1, ....., On) for “n” different height settings of the leveling bulb.

3. Record the mercury column heights in your data table for about 25 different positions of the leveling bulb. Be sure to record values which span the entire range available to you. Don’t waste any time trying to level the columns of mercury to specific height values; rather, just set the leveling bulb at a range of heights and determine the column heights as precisely as you can from the scale.

4. Check your data set for any obvious mistakes in reading the scale or recording the values.

Data Analysis You have now recorded a sizable data set which needs to be analyzed and interpreted. You will do these two things in two different stages: first, you will select a few representative values from your data set and initially examine these values to verify

Boyle’s Law (the Preliminary Data Analysis); and, second, you will analyze the full data set using the computer and Microsoft Excel.

Preliminary Data Analysis

1. Select 5 representative sets of column height pairs of readings from your data table.

These pairs of readings should cover the whole range of readings which you recorded in the full data set.

125

2. Convert your column heights to pressure in millimeters of mercury (torr) exerted on the entrapped gas. Display your results in your notebook in a small table which contains the total pressure and the volume (height in millimeters which is directly proportional to the volume). In addition to these two quantities, also list the calculated reciprocal of the pressure and the pressure-volume product.

3. Look at the 5 pairs of readings and determine the ranges of the pressure and volume which are needed to determine the axes of an XY-plot of this data. Plot the three graphs of V vs P, V vs 1/P and PV vs P where the pressure term (second term listed as in Y vs

X) is always placed on the X-axis. Be sure to label each axis with the units used.

4. What functional or algebraic forms are appropriate to these three graphs? How might you construct such a functional form to actually fit your data?

5. Do your graphs verify Boyle’s Law? How do they or do they not verify the Law?

6. Would it be possible to determine the value of the constant “k” of equations (2) and

(3)?

7. What would the volume versus pressure plot look like for a much lower temperature?

126 Complete Data Analysis

You have acquired a sizable data set for the Boyle’s Law Revisited experiment. Now use the full data set to demonstrate the application of a commercial spreadsheet program in the analysis of experimental data.

127

ROBERT BOYLE (1627-1691)

The most influential Irish-born scientist ever was Robert Boyle, who lived from 1627-

1691. He played a key role in the history of science because of his part in establishing the experimental method, on which all modern science is based. By using carefully devised experiments, Robert established the power of practical science, and knowledge took a giant leap forward.

Nicknamed "The son of the Earl of Cork and the father of Chemistry", Robert was born in Lismore, Co. Waterford, the youngest of fourteen children of Richard Boyle, First Earl of Cork. Richard, the father, came to Ireland from England to make his fortune in 1588, and got a job dealing with properties reverting to the Crown in the absence of legal heirs.

He was thus in a good position to direct some of these his own way, and he had the good sense to marry an heiress. Although imprisoned for embezzlement and theft, he managed to receive a royal pardon, and went on to accumulate a massive fortune and to advance his social standing and his political influence.

On the death of his first wife, he married the well-connected Katherine Fenton (when she was 15 and he was 37), and Robert was their youngest son. Robert was different from the other children in the family. In a brief autobiography of his early life, he paints himself as a rather self-righteous swot, preferring study to normal boyish pursuits. But his father, perhaps rather surprisingly given his own energetic career, clearly approved, for Robert

128 wrote that he was very much his father's favourite. He went to school at the famous Eton

College for a while, and then was sent at the age of 11 on a grand tour in Europe, which lasted no less than six years. He got a broad education in Europe, for he reports that, while in Florence, he was allowed to visit the famous Bordellos, though he claims he went to them out of "bare curiosity", retaining his "unblemished chastity". He was no more attracted to other diversions, for he also reported that he was somewhat rudely pressed by what he called the "preposterous courtship" of a couple of Friars, but he managed to escape from these "gowned sodomites".

His father, the Great Earl had tried to arrange for Robert to marry and live in Mallow in

Co. Cork. But his intended chose to marry her cousin, much to the relief of Robert, who was thus free to devote himself entirely to his studies. He settled in England where he carried out important work on the air pump, which he developed, and which allowed him to investigate the nature and properties of the vacuum. For example, he demonstrated that sound could not be heard in a vacuum, that a candle was extinguished, and that an unfortunate cat died.

Today's students are reminded of Robert's work when they learn "Boyle's Law", which states that, at constant temperature, the volume of a gas is inversely proportional to the pressure applied to it (V x p = constant).

Robert was a founder of the Royal Society in London in 1660, and the next year he published the most famous of his many books The Sceptical Chymist. In this, he questioned the early belief that materials were made up of four elements - earth, air, fire, and water, instead anticipating modern atomic theory. He introduced many chemical

129 tests, including the use of vegetable dyes as acid-base indicators, and flame tests to detect metals.

130 THE SPECTRUM OF A DYE MOLECULE AND THE LAMBERT-BEER LAW

INTRODUCTION

One of the most common laboratory methods of determining how much of a given chemical may be present in a sample relies on the ability of substances to absorb light. In fact, the molecule’s ability to absorb light depends on its detailed atomic/molecular electronic structure and this gives the analyst a unique tool to identify and quantify substances even within mixtures. The purpose of this exercise is to examine an electronic spectrum of a molecule and then to “discover” the Lambert-Beer Law and its practical consequences.

When a molecule absorbs a photon of light the energy of the photon is converted into molecular internal energy. Thus, it is said that when the molecule undergoes a transition from a ground state (the lowest energy level) to an excited state (a higher energy level) it undergoes absorption. The reverse of this process is known as emission. Such transitions may be illustrated with a simple energy level diagram where the molecular energy is placed on the vertical axis and horizontal bars denote the involved molecular states

Molecular Energy ¯¯¯¯¯¯¯¯¯¯¯ Excited State

absorption 89 emission

¯¯¯¯¯¯¯¯¯¯¯ Ground State

131 In the diagram arrows are shown going both upwards and downwards. We use this dual notation to cover both the cases of absorption (upward) and emission (downward) of a photon by the molecule.

The photon energy, )E (where )E = h< = hc/8), is incorporated into the molecule on absorption of light of frequency < or wavelength 8 (c is the speed of light and h is

Planck’s constant). In general, the increased molecular energy is distributed amongst the possible translational, rotational, vibrational and electronic degrees of freedom peculiar to that molecule. The number of different energy levels in a molecule is, in general, very large due to its complicated molecular electronic structure. In the case of the absorption by molecules of light in the visible-ultraviolet region of electromagnetic radiation, the internal energy changes are known to be primarily electronic in origin. In other words, the main changes occurring on absorption of such light consist of changes in the energy levels of electrons in the molecule. Hence, the absorption in this wavelength region is called “electronic absorption”. The associated graph of molecular absorption versus wavelength is called a spectrum (plural, spectra). Since the details of the energy levels of molecules depends primarily on molecular structure, the absorption of light of different wavelengths corresponding to transitions to different excited states

“fingerprints” substances.

We are familiar with the practical consequences of the molecular absorption of light. For example, dyes are identified by their characteristic colors (blue jeans are blue because the fabric dye indigo appears blue). We are much less familiar with the characteristic

132 infrared or ultraviolet absorption by molecules since we do not directly experience these events. However, we do rely on these properties in utilizing, for example, “sun screen” lotions and motion sensors.

In this exercise we will examine the spectrum of a large dye molecule, namely, crystal violet. In a subsequent experiment we will use crystal violet as a reactant in a reaction the time course of which we will study in detail.

THEORY

Consider the absorption of light of wavelength 8 by a thin “slice” of volume with differential width “dx” in which the molecular absorber has a molar concentration “c”

I0(8) 2 ¦ 2 I(8) dx

Starting on the left with the initial light intensity, I0(8), the substance in the differential slice “dx” absorbs a fraction of the light, thus, permitting only light of intensity I(8) to emerge from the other side. The relative probability that a molecule will absorb a photon of wavelength 8 is typically put on a molar concentration basis and denoted by the constant ,(8), the “molar extinction coefficient.” In quantitative terms, the differential loss of light intensity, dI, by molecular absorption, is given by

! dI(8) = I(8) @ ,(8) @ c @ dx

133 where the minus sign emphasizes that the light intensity is diminished by molecular absorption. Rearranging this equation to separate the variables I(8) and x and integrating over the limits of intensity and sample width or optical pathlength “l” gives

I(8) l ! I dI(8)/I(8) = I ,(8) @ c @ dx I0(8) 0

or ln [ I0(8)/I(8) ] = ,(8) @ c @ l

It is customary to write the molar extinction coefficient such that logarithms to the base

10 are used giving the commonly encountered Lambert-Beer Law

A(8) = log [ I0(8)/I(8) ] = ,m(8) @ c @ l

where ,m(8) is called the molar decadic extinction coefficient (often called just the molar extinction coefficient, for short) and A(8) the absorbance.

The (electronic) spectrum of a molecule consists of a plot of the absorbance versus wavelength or equivalently molar decadic extinction coefficient versus wavelength. The spectrum of crystal violet in the ultraviolet/visible wavelength region is shown below.

134

There are several features of this spectrum which merit discussion. First, the dye has been dissolved in a solvent, in this case 50/50 (v/v) ethanol/water solvent, and this means, strictly speaking, that the absorbance spectrum shown corresponds to both solvent and the dye. However, it is also known that ethanol/water solvent is transparent in this wavelength region (220-720 nm). In turn, this means that the spectrum shown above corresponds to the dye in solution and that the solvent only indirectly influences the spectrum of the dye. Second, the spectrum shows several (3) peaks; they occur at roughly

600 nm (unsymmetrical intense peak), 300 nm (small symmetrical peak) and about 240 nm (small symmetrical peak). Whether or not there is a peak at about 370 nm could be argued but we will ignore it. Clearly, these peaks have differing intensities. Third, the

135 large peak at 600 nm (594 nm to be exact) has a shoulder at about 550 nm suggesting that the unsymmetrical nature of this large peak is due to the overlap of two symmetrical peaks, one at about 600 nm and one at about 550 nm. Fourth, the spectrum seems to come to an end at about 700 nm (this is true as one could show by measuring at longer wavelengths). This fact tells us that the transition to first excited energy level of the dye occurs at 594 nm.

Suppose that we needed to measure the amount or concentration of crystal violet as a function of time without having to resort to measuring the whole spectrum of the dye at each time. Well, if we know the molar extinction coefficient ,(8) and keep the same pathlength, l, we could just make a measurement of the absorbance at one wavelength,

A(8), to determine the concentration of the dye from the Lambert-Beer law since

c = A(8)/,m(8)@l

With the great recent advances in optoelectronics, this task is quite easy to do using an experimental setup designed by Ocean Optics, Inc. We can measure the entire spectrum of our analyte in the UV/Visible range (wavelength range from 320-1000nm) and later select data at the absorption maximum for our analyte (in this case, ~590 nm).

136

Ocean Optics Experimental Setup

I I o Collected signal converted to Abs.

To PC PC2000 Spectrometer mounted on A/D card

Tungsten-Halogen Cuvette (1 cm) Single Strand UV/Vis Optical Light Source containing Analyte Collection Fiber (2 m, 400µm diameter) Silica Core, Silica clad

In this configuration, a light source (tungsten-halogen light lamp) is integrated with a sample holder made to fit a standard 1 cm disposable cuvette. Attached to this portion of the device is a fiber optic cable connecting to the spectrometer mounted on an A/D card, which fits easily into an ISA-bus slot in a PC. OOIBase is the Windows operating software for Ocean Optics spectrometers.

The light source supplies light to the sample. Light which is transmitted through the sample is then collected in the fiber, which in turn sends this light to the spectrometer.

The spectrometer measures the amount of light transmitted at each wavelength contained within the sampling window. The A/D converter, on which the spectrometer is mounted, then converts this collected analog data to a digital form and sends it to the PC. OOIBase software then takes over with basic acquisition and display functions that provide a real- time interface to several different types of signals (absorption, transmission, etc…).

137

In this experimental set-up, you will be using a type of state-of-the art fiber-optic technology which is not limited to the chemistry laboratory, but has become an integral part of our everyday lives, and continues to expand at an unheard of rate. It is almost impossible to remember our society without such conveniences as high speed communications (internet), LCD displays, etc., etc., etc.!!! Digital networks transmitting vast amounts of data at the speed of light are being built from this technology. Below is a diagram from The Times Reporter (Dover, Ohio) illustrating how these networks work

(Sources: Corning, Inc., www.howstuffworks.com, Bell Laboratories.)

138 Fiber-optics are revolutionizing many aspects of our everyday life, especially with respect to telecommunications and computer networks. Reasons for this switch from conventional metal wire (copper) include the following advantages:

1. Less expensive –optical fibers cost less to manufacture than equivalent

amounts of copper wire.

2. Thinner – optical fibers can be drawn into smaller diameters than copper

wire.

3. Higher carrying capacity – because of (2), more optical fibers can be

bundled into a given cable diameter than copper wire. More phone lines go

over the same cable or more channels come through the cable into your cable

TV box.

4. Less signal degradation – loss of signal in optical fibers is less than that in

copper wires.

5. Light signals – Unlike electrical signals in copper wires, light signals from

one fiber do not interfere with those of other fibers in the same cable. Clearer

phone conversations and TV reception (HDTV!)

139 6. Low power – because of (4), lower power transmitters can be used instead of

high voltage electrical transmitters for copper wires.

7. Digital signals – optical fibers are ideally suited for such an application

8. Non-flammable – no electricity is being passed through optical fibers, so no

fire hazard

9. Lightweight – less weight and take up less space in the ground

10. Flexible – used in digital cameras for medical imaging, plumbing, mechanical

imaging and inspection

(Sources: www.howstuffworks.com - (much more information on fiber-optics available!)

Ocean Optics, Inc., particularly CHEM2000 Operating Manual

140 PROCEDURE

In this exercise you will determine a calibration curve of absorbance versus concentration of the dye crystal violet and then use this information to determine the concentration of an unknown crystal violet solution. The first step is to make a series of solutions of the dye. Knowing the concentrations of the dye solutions and the optical pathlength, l, you can then calculate the molar decadic extinction coefficient (ε) of crystal violet from absorbance measurements of the solutions. Having determined the calibration curve for the dye you can then also determine the concentration of an unknown crystal violet solution. A secondary purpose of this experiment is to get you to fine-tune your abilities to make quantitative dilutions from a common stock solution. In doing this you will learn the techniques of pipetting solutions and making solutions to volume.

______

CAUTION! Crystal violet is an organic dye and it will stain nearly anything it comes into contact with including your skin. The chemical hazard classification of this dye is not fully determined at this time. Treat crystal violet as if it was toxic! Wear gloves and dispose of the dye solutions in the waste containers provided.

______

141 A good technique to employ when measuring the absorbance of a series of standards is to begin with the solution of lowest concentration and to work upward in concentration. Be sure to rinse the sample cuvette with a small amount of solution to be measured before making the measurement. That way, solution in the cuvette will quantitatively reflect that in the standard solution and not that from the previous measurement.

Materials

5.00 x 10-5 M crystal violet stock solution (in ethanol/water 50/50 v/v) ethanol/water 50/50 solvent volumetric flasks 50.0 mL

10 mL Mohr pipette and pump beakers to store standard solutions photometer cuvette

LED-based photometer with computer and interface disposable pipettes/bulb

IMPORTANT NOTE:

PRIOR to lab, you MUST have completed your 5 concentration calculations, including exactly how you will be preparing your solutions!!!!!!

M x V = M x V

142 Making It Work!

Step 1: Make 5 crystal violet standard solutions using a 50.0 mL volumetric flask, a 10 mL Mohr pipette and the 5.0 x 10-5 M stock dye solution. Bring each solution to volume using 50/50 v/v ethanol/water solvent. Aim for making standard solutions in the range between 1.0 x 10-7 M and 3.5 x 10-6 M. As you perfect your pipetting technique you may wish to make duplicate and/or more standard solutions to obtain a better data set. A sample detailed procedure for this might run as follows:

a. Make sure your 50 mL volumetric flask is clean. Rinse it out with distilled

water to be sure!

b. Take a clean pipette and rinse it out with small amounts of the stock dye

solution. Now practice pipetting different amounts of the stock dye solution into

an empty beaker. When you feel you can pipette volumes accurately go to the

next step!

c. Make your standard dye solutions by pipetting the required volumes of stock

dye solution into a clean empty 50 mL volumetric flask. Make each standard

solution to volume by carefully adding the solvent (50/50 ethanol/water) until the

bottom of the meniscus coincides with the volumetric’s neck marking. Stopper

and carefully invert (do not shake) the flask several times to mix the contents of

the standard solution. Finally, transfer the standard solution to a beaker for future

measurements. Clean the volumetric flask for the next standard solution.

143 Step 2: You can find the Ocean Optics software under the Start menu, Program,

OOIBase V 1.5. Using the Ocean Optics equipment, record the absorbance of each standard solution you have made along with the unknown solution(s) provided. Save each spectrum and record your file name and location. (You have a separate handout detailing how to take an absorbance spectrum.) Remember, for your reference spectrum (blank) to include all factors except your analyte.

Step 3: Using OOIBase, open each spectrum you recorded and find the Absorbance maximum using the Cursor function on the toolbar. Record each absorbance value

(remember to use the same wavelength each time!!!!). Using MS Excel, construct a spreadsheet summary of your results by entering your standard solution concentration/absorbance data for all points except the unknown(s). Graphically analyze your results by plotting Absorbance versus Concentration. Perform the linear regression analysis on the data using the spreadsheet options available to you. Calculate the molar decadic extinction coefficient (ε) of crystal violet in ethanol/water (50/50) solvent from your results.

Step 4: Determine the concentration of the unknown crystal violet solution by recording its absorbance and using your calibration curve linear regression results.

Post-Laboratory Questions:

1. Your Absorbance versus Concentration plot should be linear. Assuming that it is, what does the slope of this straight-line correspond to? From the results of the linear regression

144 analysis of your data determine the best value of the extinction coefficient. What is/should the intercept of your calibration curve be? Discuss any difference from your expected result.

2. If the Absorbance versus Concentration plot were not linear even after you had checked it by duplicating your measurements, what might this suggest about the dye in solution?

3. What would be the result of picking a light emitting diode (LED) which emitted light only at, say, 300 nm? 700 nm?

4. The dye crystal violet is, indeed, purple in color to the eye. From the absorbance spectrum given in the handout, determine why this is so.

Protocol for using Ocean Optics Equipment

Consult the Ocean Optics, Inc. CHEM2000 Operating Manual Version 1.1 for specific instruction on acquiring absorbance measurements (pg.15), understanding integration time (pg. 10), and deciphering the toolbar (pg. 11).

145 Acid-Base Titrations

INTRODUCTION

The laboratory titration of acids and bases is prominent in the chemist’s arsenal of analytical procedures aimed at identifying and quantifying the amounts of these important substances. The purpose of this experiment is to familiarize you with the titration of strong, weak and polyprotic acids.

Through your experience in lecture you have already become familiar with the properties of acids and bases in aqueous solution. Before this laboratory experiment you should review text pages and/or class notes as follows:

Oxtoby and Nachtrieb

(Ch. 10) pp. 314-353

In this laboratory we will simulate titrations of three dilute acid solutions containing a strong (hydrochloric), a weak (acetic) and a polyprotic (phosphoric) acid using software resident on laboratory computers. Subsequently, you will perform the actual titrations and compare your computational results with those obtained from actual experiment.

PROCEDURES

There are 2 distinct phases of this laboratory: (1) Computer Simulation and (2)

Experimental Titrations.

146 COMPUTER SIMULATION

The laboratory computers have a software package written to help you understand acidbase titrations. Specific programs of interest to us are ACIDBASE/TITRATE, FABTIC, and ALPHA. You will examine each of these programs and generate data files containing your computational results for selected acids and bases which can be loaded into Microsoft Excel for analysis and plotting.

* * * STORE FILES TO YOUR DISK AND RECORD YOUR FILE NAMES * * *

Acid-Base Pairs Examined:

0.050 M hydrochloric acid (HCl) titrated with 0.100 M sodium hydroxide (NaOH)

0.050 M acetic acid (CH3COOH) titrated with 0.100 M NaOH

0.050 M phosphoric acid (H3PO4) titrated with 0.300 M NaOH

ACIDBASE/TITRATE

This program is found by operating your computer in MS-DOS mode.

C: cd acidbase - Enter

C: acidbase/titrate - Enter

Follow the menu-driven paths examining the details of titrations for the three acid/base pairs, with conditions the same as those you will use in the laboratory (see details above if you forget). You should not correct for ionic strength in your initial simulations. In this program you will “see” the titration occur and plot simultaneously. Pay attention! This

147 is what you should expect when you perform these titrations! No data will be saved from this program.

Additionally, try out the choice “Use an unknown acid or base from the list”. Identify at least 2 different unknowns using the data provided. Record your “code” number, your identification and how you came to your conclusion.

FABTIC

This program is found by operating your computer in MS-DOS mode.

C: cd acidbase/ - Enter

C: acidbase/acidbase - Enter

Choose FABTIC from the initial menu (choice #1)

You will calculate an acid-base curve (choice #1)

Use FABTIC to simulate the specific titrations you will do in the laboratory, being sure to use the same volumes and acid/base concentrations (see above). Do not correct for ionic strength in your simulations. Store the results in suitably labeled files (for example,

ACETIC.TIC - USE SHORT LABELS) on your data diskette for later use with Excel.

You should finish with 3 files of generated numerical values representing a titration curve for each acid/base pair.

ALPHA

C: cd acidbase/ - Enter

C: acidbase/acidbase - Enter

Choose ALPHA from the initial menu (choice #4)

148 You will enter a file name and then run ALPHA (choice #2)

Follow menu driven paths and use ALPHA to calculate the fraction of the various chemical species present in each titration. Follow their behavior and prevalence over the course of the titration. Use suggested default values given by the program. Store the results of these calculations on your data diskette using suitably chosen filenames (for example, HCL – USE SHORT LABELS) for later use with Excel.

You will finish with multiple files for each acid/base pair. Put together in Excel, this generated data will represent your alpha curve for each acid/base titration.

Analysis of Computational Results Using Microsoft Excel

Import your results from FABTIC and ALPHA into the spreadsheet. When importing these files use the “comma delimited” file selection in Excel.

from FABTIC -

Plot the titration curves for each acid solution. Orient pH on the Y-axis and Titrant

Volume on the X-axis. Identify the pKa values on your plots and the equivalence points in each titration.

from ALPHA -

Plot Fraction versus pH for all three acid solutions. Orient Fraction on the Y-axis and pH on the X-axis. On your plots identify the particular chemical species in each solution as a function of pH. From the ALPHA plots identify the pKa values for each species involved.

149 Additionally –

Plot by hand a rough graph detailing a titration curve with its corresponding ALPHA plot superimposed for one of your acid/base pairs. How does the ALPHA plot relate to the

FABTIC titration curve? Using the ALPHA software results, determine where on your titration curves the different species exist. Mark them on the FABTIC portion of the plot.

In summary, you should have acquired three FABTIC titration curves and three

ALPHA plots, along with one hand drawn plot of your own, all with appropriate points identified (pKa, equivalence point, species existence). Mount these curves in your notebook for use later.

EXPERIMENTAL TITRATIONS

You will be titrating with the aid of Labworks Software and Equipment. The labworks software program may be accessed by clicking on the Shortcut to Labworks icon found on the desktop of your computer.

Titrate 0.050 M solutions hydrochloric acid (HCl), Acetic acid (CH3COOH or HAc), and phosphoric acid (H3PO4) (25.0 mL of each) with the standardized NaOH solutions provided. Do this in steps as follows:

150 1. Open Labworks II v. 4.0 (Shortcut on desktop). Choose Calibrate, and calibrate the pH electrode using standard buffer solutions provided and following software menu instructions. A relatively stable reading is fine prior to typing in the buffer pH.

2. Rinse out your freshly cleaned buret with the base solution you will use in the titration.

Refill the buret with freshly prepared base solution (supplied by your instructor). Always begin with a full buret to eliminate the need to refill during the course of the titration.

3. Pipet 25.0 mL of the acid solution to be titrated into a small beaker (250 mL size is adequate for this purpose) to which you can add enough distilled water to bring the final liquid level above the electrode’s measuring port. (See you instructor for details.)

4. Put a magnetic stirring bar and the calibrated pH electrode in the beaker of acid solution and place them on top of the magnetic stirring machine. Activate the stirring motor so as to have a slow, steady stirring rate. Be sure drops from the buret will fall directly into the beaker with minimal splashing, and if necessary, through the drop counter apparatus.

151 pH Titration Experimental Set-Up

Buret containing base solution

To PC – Labworks Interface Drop counter (may or may not be present)

pH electrode

Beaker containing acid solution and stir bar

Magnetic Stir Plate

5. Under FILE, OPEN, open TITRATE.EXP and click on ACQUIRE.

Here’s where the teamwork really begins!

6. Click on START and check to make sure the X-switch is in the ON position (UP).

• Entry desired is the initial buret reading.

• When hitting OK for this entry – you MUST simultaneously start the buret flow

at a steady rate (~1-2 drops/second) *

7. When the titration is complete (pH has sufficiently leveled off), simultaneously flip the X-switch to the OFF position (down) and stop the buret flow.

152 • Entry desired is the final buret reading

8. SAVE DATA!!!!

9. Repeat this titration procedure as needed to titrate the remaining samples of acids being sure to note two different concentrations of base needed in your three titrations.

Everyone should do at least one titration using the drop counter.

10. Transfer (Import) your results to Excel and plot the titration curves for comparison with your simulated curves for these same acids. Enter all of these curves in your laboratory notebook.

* IMPORTANT POINT *

The computer digitizes each titration curve by recording the pH as a function of the

TIME elapsed during the titration rather than the volume of titrant added. However, since you have set a constant drop rate from the buret the TIME during the titration is directly proportional to the total number of drops added and thus to the volume of titrant added.

153 POST-LAB QUESTIONS

1. How do your experimental titration curves compare with those obtained using

FABTIC (curve shape and orientation, estimated pKa values and equivalence points where possible to determine)? Note the similarities and differences between them and offer explanations for the differences. Choose one titration curve and summarize in words and/or pictures what is happening along each step of the way. Include details about where and when certain species exist.

2. The pKa of HCl is about !5. What does this mean? Is it possible to have an acid solution so concentrated so as to have a pH of !5?

3. Look carefully at the results for phosphoric acid. This acid has three protons which ionize during the titration. Note the positions of the first two pKa values on the titration curves and ALPHA plots. Where is the third such value? Why isn’t this value obvious in the results?

4. Time recorded for each titration correlates to volume of titrant added. Consider your titration(s) which utilized a drop counter. Total number of drops similarly can correlate to both volume and time. Calculate the volume of titrant per drop (assuming uniform drop size). Also calculate your steady-state drop rate.

154 YOUR ASSIGNMENT:

SPECTROPHOTOMETRIC DETERMINATION

OF AN EQUILIBRIUM CONSTANT

PROJECT DESCRIPTION:

As per patent #JP60178351, ferrous thiocyanate, Fe(SCN)+2, is an integral component of a peroxide detection paper used to confirm the formation of peroxide in gasoline.

The paper is produced using a process such as this:

+ Vacuum Drying Coat with an inert Paper organic substance +2 Fe(SCN) (aq) insoluble in gasoline Test Paper

Peroxide present in gasoline will react with ferrous thiocyanate present in the test paper and form ferric thiocyanate, Fe(SCN)+3 . Ferric thiocyanate may then be detected.

155

Production of this paper is what we’re interested in. We want to know how to make an optimized solution of Fe(SCN)+2 . We will produce Fe(SCN)+2 from iron (III), Fe+3 , and thiocyanic acid, HSCN. The objective of this experiment is to determine the equilibrium constant, K, governing the formation of Fe(SCN)+2 from iron(III) and thiocyanic acid by using a spectrophotometer to measure the concentration of Fe(SCN)+2.

BACKGROUND

Every chemical reaction is subject to equilibrium conditions that govern the concentrations of the products and reactants. In this experiment, known quantities of iron (III) and thiocyanic acid are mixed to form a dark red iron-thiocyanate complex.

The balanced reaction is:

+2 + Fe+3 + HSCN Fe(SCN) + H (aq) (aq) (aq) (aq)

At equilibrium, the concentrations of the products and reactants are governed by the mathematical expression:

[FeSCN+2 ][H + ] K= [Fe+3 ][HSCN]

where K is called the equilibrium constant and the square brackets denote concentrations in moles per liter (M). K is not a true constant in that it depends on temperature, and it is more exact to use “activities” in place of concentrations. The equation is, however, quite satisfactory when temperature is controlled at a constant value. As a rule of thumb, it is

156 not unusual to find that the equilibrium constant doubles or triples when temperature changes by ten degrees Celsius.

However, the expression for K is not quite so simple! Thiocyanic acid is a weak acid and subject to the equilibrium:

+ - HSCN H + SCN (aq) (aq) (aq)

[H+- ][SCN ] = 1.4 x10-2 at 25 o C [HSCN]

Thus, when [H+] becomes small, the formation of thiocyanate as well as iron(III) hydroxide complexes have to be considered as part of the equilibrium problem. In this experiment, these very interesting problems are avoided by adding strong acid to solution so that concentrations are governed by only one equilibrium expression.

Fe(SCN)+2 is a dark red cation that absorbs light most effectively at 447 nm

3 -1 -1 (ε447 = 4.70 x 10 M cm ). Thus, a spectrophotometer can be used to measure the equilibrium concentration of the Fe(SCN)+2 cation (REMEMBER BEER’S

LAW????!!!!). The equilibrium concentrations of H+, HSCN, and Fe+3 can be calculated from the initial quantities, stoichiometry, and the equilibrium concentration of

Fe(SCN)+2.

PRIOR TO BEGINNING THIS LAB, you MUST work out a method to calculate K using the equation described earlier in the text. Remember, in the K equation, concentrations are expressed in the equilibrium state!

157 Additionally, you should outline your protocol including how you will be making your solutions, what their composition is, how you’ll be making your measurements, and a data table where you can record pertinent information.

YOUR PROTOCOL MUST BE APPROVED BEFORE BEGINNING THIS LAB!

EXPERIMENTAL PROCEDURE

You will actually be performing the reaction which creates the Fe(SCN)+2 cation. This should be repeated a minimum of 4 times to significantly reduce experimental error and give comparable data sets. You will again be utilizing the Ocean Optics spectrophotometer to perform your measurements of absorbance for the Fe(SCN)+2 cation. Make at least four solutions, complete at least 4 trials, and obtain at least 4 calculated values for the equilibrium constant, K.

Here is where your ability to experiment comes in! Create your own procedure (be careful to account for all variables). Below is an outline of a procedure you might model your own experiment after. You should repeat your process a minimum of 4 times.

158 TRIAL X

HCl (provided at 2.0M) Set volume of 2.0mL

Measure Abs. using Ocean Optics equipment. KSCN

(provided at 0.0033M) 10.0 mL solution containing Volume between 2.0 and 6.0mL Fe(SCN)+2 Mix thoroughly! Avoid mixing air into solution

to eliminate reaction with O2 or CO2.

Fe+3 (provided at 0.0033M) Add to bring total volume to 10.0 mL

That procedure could also be detailed in tabular format:

2.0 M HCl 0.0033 M KSCN .0033 M Fe+3 Fe(SCN)+2 Volume used Volume used Volume Used Total Solution Volume Trial Number (mL) (mL) (mL) (mL) between 2.0 and 1 2.0 6.0 ? 10.0 between 2.0 and 2 2.0 6.0 ? 10.0 between 2.0 and 3 2.0 6.0 ? 10.0 between 2.0 and 4 2.0 6.0 ? 10.0

Each trial should use different volumes of KSCN and Fe+3.

159 Remember, this tests your ability to make both accurate and precise solutions – make sure you measure volumes needed in the best equipment possible (i.e. don’t use a beaker to measure out 2.0 mL of solution).

When using Ocean Optics equipment, refer to procedure from The Spectrum of a

Molecule and the Lambert-Beer Law. What should be your blank for the reference spectrum?

DATA ANALYSIS

After obtaining Absorbance values for Fe(SCN)+2 , check to make sure you’ve recorded all data necessary to complete your calculation for K.

Organize your results! Show one sample calculation of K, and all 4 K values you obtained. Also calculate an average value for K.

Present your results in the format of a short report you would submit to your superior.

Be creative! You need to communicate your results, why the procedure you used to get them is scientifically acceptable, and what to make of all the numbers you’ve come up with – in short, make sense of everything for someone who is not as well versed in chemistry as you!

160 POST-LABORATORY QUESTIONS

1. What effect does a dirty cuvette (caused by fingerprints, water spots, or lint) have on the absorbance reading for Fe(SCN)+2 solution? Does this error cause the reported value of K to be too high or too low? Explain?

2. Compare the K values for different trials. Assuming a constant temperature, how would you expect the K values to compare?

3. Compare the absorbance values of your four samples. Do the values suggest the presence of a limiting reagent in the initial solution? Explain your answer.

4. Discuss your absorbance values in relation to the concentrations of Fe(SCN)+2 . Can you find a relationship? Explain. (A plot from your data might help here).

5. In this lab we used KSCN as one of our chemical components. In the background information, the Keq equation shows the reaction using HSCN. What difference does replacing HSCN with KSCN make, if any? Explain.

161 SYNTHESIS AND PROPERTIES OF SOLID

SOLUTIONS : ALUMS

INTRODUCTION

The name “alum” suggests that such a substance must contain aluminum in its composition. A common example of an alum, NaAl(SO4)2@12H2O, can be is found as an additive to baking powders. In general, alums are a class of inorganic compounds

+ 3+ + corresponding to the general formula M M (SO4)2@12H2O, where M corresponds to a

+ + + + 3+ monatomic cation (for example, Na , K , Ag ) or NH4 and M is a monatomic trivalent cation which can be from the main block metals (most notably Al3+) or from the transition metals (for example, Cr3+, Mn3+, Fe3+, Co3+). An alum is also a hydrate of specific proportions which derives from the fact that the 12 water molecules occupy specific positions in the solid phase structure (crystal phase) of the compound. Alums are even more interesting in that they also form solid solutions of variable proportion. They do this by varying the extent of substitution of M3+ (see subsequent discussion).

The purpose of this rather lengthy experiment is to allow you to explore the synthesis and purification of alums and to teach you to formalize your results in a presentation to the class. Since alums are good examples of solid solutions they will bridge the gaps among lecture topics: solutions, equilibrium, and the solid state.

This experiment will be done over several weeks of time. In this experiment you will learn several very important techniques available to the laboratory chemist. First, you will

162 synthesize, on a preparative (large) scale, the common alum KAl(SO4)2@12H2O. (You did this last quarter in Chem 111 on a smaller scale.) Second, you will learn how to manipulate solution/solid equilibria to achieve purification by crystallization. You will also learn how to grow beautifully large single crystals. Third, you will formulate a series of solid solutions (chrome alums) of your own choice and then crystallize and characterize their properties. Fourth, you will select an independent experiment to do with alums. Finally, you will report your findings to the class in a formal presentation. In doing these five things you will experience the overall process that professionals in any area of science go through in laboratory research work.

PROCEDURE

This experiment consists of two distinct parts. In Part One, you will synthesize

KAl(SO4)2@12H2O from solid aluminum metal foil and then crystallize this material to purify it. In Part Two you will synthesize a series of solid chrome alum solutions which you will then crystallize and characterize.

Part One: Synthesis of starting materials

Metallic aluminum dissolves in strong aqueous base to form hydrogen gas and the

! Al(OH)4 ion in solution:

2 Al + 2 KOH + 6 H2O º 2 KAl(OH)4 + 3 H2 or

! ! 2 Al + 2 OH + 6 H2O º 2 Al(OH)4 + 3 H2

163 Addition of H2SO4 to this solution will both neutralize the excess KOH and convert the

! Al(OH)4 ion to the insoluble aluminum hydroxide:

2 KAl(OH)4 + H2SO4 º 2 Al(OH)3 + K2SO4 + 2 H2O or

! 2 Al(OH)4 + 2 H+ º 2 Al(OH)3 + 2 H2O

The insoluble aluminum hydroxide is finally neutralized by addition of excess sulfuric acid to form aluminum sulfate which is a soluble ionic compound:

2 Al(OH)3 + 3 H2SO4 º Al2(SO4)3 + 6 H2O or

+ 3+ 2 Al(OH)3 + 6 H º 2 Al + 6 H2O

Finally, as the solution of aluminum sulfate containing potassium ions is cooled, the double salt potassium aluminum sulfate crystallizes out of solution as the hydrate:

+ 3+ !! K + Al + 2 SO4 + 12 H2O º KAl (SO4)2 @ 12 H2O

(A double salt is an ionic compound that contains two different cations or two different anions.)

The experimental protocol is as follows:

164 Step 1: Dissolution of aluminum metal in base

Weigh out 2.5 g of aluminum foil to 0.002 g and after cutting it into small pieces place it in a clean dry 500 mL beaker. To this beaker add 50 mL of 3 M KOH. Carefully take the beaker immediately to the hood since hydrogen gas will begin to evolve. In fact, this oxidation (of aluminum metal) reaction is strongly exothermic and the beaker will become progressively hotter as the reaction proceeds. Do not handle the beaker since it will become so hot as to burn you! After the hydrogen gas stops issuing from the beaker and it has become cool enough to handle take the beaker to your laboratory bench again.

If necessary cool the beaker further with running cold water until you can handle the beaker easily. If the solution in the beaker is completely clear continue with the next step in the synthesis. If the solution is not completely clear filter the solution by gravity filtration into another beaker. (Directions for gravity filtration may be found on p. 12 of

Alexander and Steffel, “Chemistry in the Laboratory.”) You can dispose of the filter paper by putting it into the trash container.

! 3+ Step 2: Conversion of Al(OH)4 to Al by reaction with H2SO4

To the clear solution in the beaker add 45 mL of 6 M H2SO4 a few milliliters at a time.

After each addition swirl the liquid to mix the contents. In the beginning, solid Al(OH)3 will precipitate but in the end it will dissolve as the acid neutralizes the metal hydroxide completely. After you finish adding the sulfuric acid, gently warm the beaker to just below boiling temperature. If you have done this procedure correctly all the solid will

165 have dissolved and the solution will be clear. If all of the solid does not dissolve carefully add a few more drops of sulfuric acid - but not more than about a milliliter total volume.

Step 3: Precipitation of KAl (SO4)2 @ 12 H2O by cooling

Cool the beaker by first placing it under running cold water and then in an ice bath. To entice crystals of the solid to form you might gently stir the solution in the beginning.

While you continue to wait for the crystallization process to go to completion prepare a gravity filtration apparatus for the next step.

Step 4: Harvesting the solid KAl (SO4)2 @ 12 H2O

After allowing the solid alum collect on the bottom of the beaker, decant as much of the clear liquid (mother liquor) from the flask as possible without pouring off any crystalline material. To the remaining solid in the beaker add about 25 mL of ice-cold distilled water, swirl to mix the contents, and allow the solid to settle out again. Decant as much of the mother liquor as possible again without pouring off any crystalline material. Repeat this washing process until the decanted mother liquor tests neutral with the indicator thymol blue. This will probably require several washings so be sure to carefully decant the clear liquid each time to maximize the solid remaining at the end! As a final step transfer the solid alum to the filtration apparatus and wash the crystals three times with about 10 mL of ice-cold acetone to remove any remaining water.

166 Transfer the crystalline material to a large piece of aluminum foil where you should finely spread the material out to air dry. After the solid is completely dry transfer it to a small beaker which has been previously weighed, weigh the contents, and from the reaction stoichiometry determine the percent yield for the reactions you have carried out starting from aluminum metal.

167 SYNTHESIS AND PROPERTIES OF SOLID

SOLUTIONS : ALUMS

Part Two: Making crystals of alum and alum solid solutions

The second part of this experiment is described in the Companion (“Teaching General

Chemistry, A Material Science Companion,” A. E. Ellis, M. J. Geselbracht, B. J.

Johnson, G. C. Lisensky and W. R. Robinson, American Chemical Society, Washington,

DC (1993), pp 372-376).

______

NOTICE: At the end of the experiment you should select one of the Open-ended

Experiments which are listed. After you select the one you wish to do, consult your instructor concerning the procedure which you will use. Be sure to carefully document each step of your experiment in your notebook.

______

Additional Notes:

1. In making up the chrome alum solid solutions we will make a series of chrome/potassium alum crystals. For example,

100/0 parts by volume

90/10

168 80/20

70/30

60/40

50/50

40/60

30/70

20/80

10/90

0/100

Your instructors will organize this effort and determine which students will make which solid solutions.

At the end of this experiment the whole lab group will convene and present the results of these crystal growth experiments. A 2-4 minute oral presentation by each student will be made and a group discussion of the results will follow.

Please reference the following for laboratory procedures:

Ellis, A.B.; Geselbracht, M.J.; Johnson, B.J.; Lisensky, G.C.; Robinson, W.R.

Experiment 3: Solid Solutions with the Alum Structure. Teaching General Chemistry: A

Materials Science Companion, 1st Edition; American Chemical Society: Washington,

D.C., 1993; 373-375.

169 Polymers, Gels, Ion-Exchange, and Stuff Like That

Introduction

Polymeric materials are of immense importance in our everyday lives. Life itself depends on polymers of nucleotides, amino acids and sugars for its very existence. Plastics are polymeric materials used in everything from automobiles to computers. So it is important as we learn about materials that we learn something about polymers.

What is a polymer?

The simplest of all macromolecules are synthetic linear (chain-like) polymers which consist of long unbranched chains of small identical subunits, or sometimes, of two or three different kinds of subunits. They are formed by polymerization of simple substances known as monomers. For example, a very important class of polymers are those based on the substituted vinyl monomer as shown below. The “X” indicates the position of substitution on carbon which defines different types of vinyl monomers.

H H H H C H C H C C H C H C H

X Cl

vinyl-X vinyl chloride

styrene

170 Corresponding to these monomers are the polymers poly(vinyl chloride) and polystyrene both of which are of tremendous commercial importance (were talking about plumbing and drinking cups here!). More examples of these polymers are

polyethylene

H2 H2 H2 H2 H2 C C C C C C C C C H2 H2 H2 H2 poly(vinyl alcohol)

H2 H2 H2 H2 C C C C CH CH CH

OH OH OH

It is important to know that these linear polymers have a three dimensional structure. In the case of polyethylene we could denote this by drawing the structure in perspective as

HHHHHHHH

CCCC CCCC H H H H HHHH

Long chain molecules of this type can be prepared with almost any chain length - from just a few to more than a million subunits or monomeric units! If the subunit of a linear polymer contains an ionic group, the polymer is known as a polyelectrolyte. Thus, the linear polymer poly(acrylic acid)

H 2 H 2 H 2 H 2 C C C C CH CH CH

COOH COOH COOH

171 is a polyelectrolyte since the side-chain –COOH is capable of ionizing to the carboxylate,

–COO–.

H 2 H 2 H 2 H 2 C C C C CH CH CH

COO- COO- COO-

If the linear polymer contains side-chains which are chemically reactive cross-linking of the polymer will form a network of linked polymer chains. For example, poly(vinyl alcohol) can be cross-linked with a dialdehyde, such as glutaraldehyde, OHC-(CH2)3-

CHO, which reacts with two poly(vinyl alcohol) chains to link them by forming two 6- membered acetal linkages.

H2 H2 H2 H2 C C C C CH CH CH

OH OH OH H2 H2 H2 H2 C C C C CH CH CH O H C OOOH + CH CH2 H3O

H2C CH2

CH2 CH2 glutaraldehyde glutaraldehyde CH 2 cross-link C CH H O O O HO OH OH OH CH CH CH C C C C CH CH CH H H2 H H2 2 C C C C 2 H H H 2 H2 2 2

172

Cross-linked networks can be used to entrap other polymer molecules to make a polymer blend.

For example, NASA has developed a blend of cross-linked poly(vinyl alcohol) and poly(acrylic acid) for use in space batteries. This blend when used at an appropriate pH can ion-exchange cations as shown below.

Monocation Exchange

H 2 C H 2 C CH COO- CH COO- H 2 C H 2 C CH COO- CH COO- + M+ H 2 C H 2 C CH COO- M+ CH COO- H 2 C H 2 C

Dication Exchange

H 2 C H C 2 CH COO- CH COO- H 2 C H C 2 CH COO- 2+ CH COO- + M 2+ H 2 C M H C 2 CH COO- CH COO- H 2 C H 2 C

For example, this blend absorbs Cu2+ ions from solution with great affinity, that is, it incorporates the copper cations by exchanging the polymer’s cations (typically H+ or

Na+) for Cu2+ ions. In fact, this blend is now being used by NASA scientists as a “cleanup” material for spills of toxic cations in the environment. Most household “water

173 softeners” are made with ion-exchange materials (resins) which exchange calcium and iron in the water with the sodium or potassium ions of the resin.

What is a gel?

Gels in most cases consist of two components: a solid phase (like gelatin) and a liquid phase (like water). Jelly is the common name for a gel although this term is technically restricted to elastic (so-called “linear”) gels. We commonly associate the property of elasticity with jellies (remember Jello!). Gelation is the process of forming a gel. The familiar table jellies (blackberry jelly + toast = mmmmm!) are examples of gels prepared from fruit pectins (complex carbohydrate polymers) which “gel” in the presence of water.

Many famous French desserts (dream of mousse au chocolat! mmmm!!) are made with gelatin, which is animal structural protein gotten from horn, hoof and cartilage, gels spontaneously in the presence of water. The term xerogel is used to describe dry gels (for example, a sheet of gelatin). Xerogels usually contain less liquid than solid whereas gels contain more liquid than solid. Some gels contain more than 99% liquid! Gels may be classified as either inorganic or organic depending on the composition. Gels with water as the liquid component are called hydrogels. According to this classification, blackberry jelly is an organic hydrogel. Gels containing an organic liquid are called organogels.

Some commercial paint strippers are sold as organogels containing chlorinated solvents which do the actual “stripping” of paint. In this case the gel binds the organic liquid which by itself has a very high vapor pressure and disappears quickly by evaporation.

The structure of gels has been of interest for decades due to the commercial importance of gels. From a variety of methods of analysis it is now known that gels are networks in

174 which the liquid phase is bound to fibrous or chain-like particles as well as being mechanically confined within the pores of the network structure. A macroscopic analogy would be a sponge. Transdermal patches are network solids with drugs sorbed into their networks. With time such patches transfer the sorbed drug across the epidermis/dermis and into the bloodstream.

In this experiment we are going to make a network solid which acts as a hydrogel. This will be done by cross-linking poly(vinyl alcohol) with glutaraldehyde in the presence of

+ the catalyst H3O . By varying the amount of cross-linking we will make hydrogels with differing swelling capacities. We will also make a network solid with an entrapped polyelectrolyte which acts as an ion-exchange medium for cations. To help describe these hydrogels we define the swelling index, SWI, as

SWI = Volume(hydrated)/Volume(dry)

We will measure this index for several hydrogels which we make in this experiment.

Experimental Details

In this experiment you will prepare cross-linked poly(vinyl alcohol) in several ways.

First, you will make these networks with varying amounts of the cross-linking reagent, glutaraldehyde, and catalyst, HCl. After drying these samples, you will measure the volume of the xerogel and its change on rehydration of the network to form a hydrogel.

Second, you will prepare a blend consisting of cross-linked poly(vinyl alcohol) with entrapped poly(acrylic acid). This blend will then be used to absorb a cation from solution by ion exchange.

175 Experimental Procedure

Lab Day 1 All calculations to be done prior to laboratory

First Preparation (Individual) – Investigation of Physical Properties of Networks

You will be preparing several networks with varying amounts of cross-linking reagent, glutaraldehyde, and catalyst HCl. The general recipe is as follows:

3.00 mL 10% poly(vinyl alcohol), “PVA”

0.30 mL 5% glutaraldehyde

0.30 mL 0.5 M HCl

1.20 mL H2O

______

4.80 mL total volume

A. Investigate the effect glutaraldehyde has on the structure and properties of the

network

One of your networks should follow the recipe exactly.

Make four more networks, varying the amount of glutaraldehyde used in each. Keep the volume of glutaraldehyde between 0.10 mL and 0.70 mL. Adjust the volume of H2O to keep a constant final volume of 4.80 mL.

176 B. Investigate the effect HCl has on the structure and properties of the network

For the final two networks, double the amount of the catalyst HCl to 0.60 mL in any two of your recipes previously prepared. Again adjust the amount of H2O to keep the final volume of 4.80 mL.

Have all of your recipes written out in your laboratory binder prior to laboratory.

Use the following protocol to prepare your networks:

1. In a small plastic petri dish (35 x 10 mm) add 3.00 mL of 10% PVA

Note PVA solution is quite viscous and difficult to measure. To overcome the

difficulty involved in accurately pipeting this liquid you can use the following

trick. Using a pipet, accurately dispense 3.00 mL of H2O into a small beaker.

Mark the outside of the beaker using a pen with a line to indicate the volume

(bottom of the meniscus). Empty the beaker of the water and replace it with PVA

solution to match the volume of water. Now pour the PVA solution into your petri

dish being sure to scrape all of the PVA solution out of the beaker.

2. Your teaching assistant or instructor will dispense the appropriate amount of glutaraldehyde solution into your petri dishes. Be sure to know how much glutaraldehyde solution is to be placed in each of your petri dishes. Label each of these petri dishes with a marking pen to keep track of them.

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3. Dispense the appropriate volumes of H2O into your petri dish using a 1.00 mL pipet.

4. Using a glass stirring rod, mix the solutions in the petri dishes thoroughly until the mixtures are completely homogeneous.

5. HCl is the catalyst for the cross-linking reaction and is added last. Dispense the proper amount of HCl using a 1.00 mL pipet into each petri dish. Immediately stir the solutions with a glass rod until they are homogeneous.

6. Cover the petri dishes with tops and place aside to let them gel – APPROPRIATELY

LABEL AND KEEP TRACK OF YOUR DISHES!!!

Second Preparation (In pairs) – Investigation of Absorptive Properties of Networks

PVA polymer blended with a polyelectrolyte:

1. Choose a laboratory partner. With this partner make a blended polymer disc in a large plastic petri dish with the following composition:

15.0 mL 10% PVA

15.0 mL 10% PAA (poly(acrylic acid))

1.50 mL 5% glutaraldehyde

1.50 mL 0.5 M HCl

+ PAN dye solution from the instructor

178 Be sure to prepare this polymer disc in the same way you did those containing PVA only.

In the first step mix the volumes of the two polymers together stirring until the resulting viscous solution is completely homogeneous.

Cover and set the petri dish aside for the polymer blend to cure for at least one day followed by drying to form the xerogel.

2. Repeat this same preparation without the added PAN dye solution

179 More about Poly(vinyl alcohol) – poly(acrylic acid) blends

You have made two large films containing poly(acrylic acid). These films are cation exchangers and, as such, can sorb a variety of cations into them. For example, these films can be used as a sensitive test for Cu2+ ion in drinking water because they concentrate this ion even from very low concentration solutions. This test relies on the fact that Cu2+ ion itself has a beautiful blue color to it arising from its d-electron valence structure.

One of the films you have made contains the dye “PAN” which stands for

1-(2-pyridylazo)-2-naphthol. This dye has the structure

N N

N N N N M2+

OH O H

Free PAN Structure Chelated PAN Structure

and binds divalent cations by “chelating” with the cation using the exocyclic oxygen and nitrogen atoms. The dye itself carries no excess charge and thus the chelate is also a cation. These properties can be nicely combined into a colorimetric film test for cations as you are about to find out.

180 Your mission should you choose to accept it (be sure to document all your work in your notebook) will encompass the following:

1. Determination of the colorimetric responses of the films with and without PAN to the collection of cation solutions provided to you.

2. In pairs, try to design a test for the simple unique colorimetric identification of cations.

You might start this exercise by thinking about the following

a. What cations give unique colors in either of the films?

b. What cations interfere with each other in the test? In other words, does one

cation prevent you from detecting another cation? Can you sorb one cation into

the network, then place the loaded film into another cation solution and determine

both cations?

c. Is there a sequence of tests that you can do to determine at least some of the

cations? (Suppose you were given an unknown cation solution (one ion). How

would you tell what it was? You could ask a fellow lab student to give you an

“unknown” to test for.)

181 Lab Day 2 Observations MUST be thorough

First Preparation – Varied glutaraldehyde and HCl:

1. With a ruler containing millimeter graduations measure the diameters and thicknesses of your dried polymer samples (xerogels). Estimate the thicknesses and the diameters as best you can which should be to about ± 0.5 mm. From your measurements calculate the xerogel volumes.

2. Rehydrate each xerogel by placing each sample in a volume (at least 20 mL) of distilled water and waiting until the swelling of each sample has stopped. (Note that this may be more than one laboratory period and that you will need to make arrangements with the instructors to do this.) Measure the thicknesses and diameters of each gel and calculate the volumes of the rehydrated polymer gels.

3. Calculate the swelling indices for each gel disc – Summarize your data and observations (A table might be useful!). Be sure to note in observations what effects the variations in formulations you used have on the swelling indices and on the disc sizes in general.

Second Preparation – PVA-PAA and the effect of doping:

1. Rehydrate both cured polymer blend discs and cut them into several, approximately 1 cm x 2 cm, pieces with a razor blade. You need not be exact in cutting these strips of the blend.

182 2. Take one 1 cm x 2 cm piece of the wet polymer blend and rinse it in distilled water and then blot the excess water on the blend with a piece of paper. Now place the blend piece in 30 mL of Cu++ solution which you have placed in a small screw-cap vial provided for you. After a few minutes take a piece of litmus paper and measure the pH of this water solution and record your measurement. Note the color of the polymer blend before and after incubating it in this solution for a day or two and up to several days.

3. Take another piece of the wet polymer blend and follow the same procedure above.

After an initial pH measurement, adjust the pH using either an acidic or basic solution.

Observe what happens. Make sure to investigate both acidic and basic pH adjustments.

4. With the remaining pieces of the blend you can choose other cation solutions (Co++,

Fe+++, Ni++) to test your blend strips with in the same way you did for Cu++ in solution.

Be sure to note the pH of each solution which you test with the polymer blend in the same manner as described above. You might try your own variation on this test for cations after you discuss your procedure with the instructor.

5. Repeat this same experiment with the ion solutions with PAN dye-loaded film pieces.

6. Complete your independent test using the PAN dye-loaded film pieces.

183 Post-Laboratory Operations

1. Make a table of your results for the cross-linked poly(vinyl alcohol) discs. Tabulate the thicknesses, diameters, volumes and swelling indices which you measured. What effect does the amount of glutaraldehyde added have on the discs? What about the amount of catalyst, HCl? Draw some basic conclusions from your results.

2. Tabulate your results for the polymer blend/cation solution experiments. What conclusions can you make from your results? What is the influence of pH on the results?

What is the chemistry involved in this effect (write chemical equations for this chemistry)? What effect does PAN dye doping of films have on your results?

3. Summarize the effectiveness of your independently designed cation test experiment – what can you conclude from this test?

184 Honors Chemistry Laboratory

Crystal Model Building Experiments

Part I. Using the solid state model kit (ssmk), build the following structures.

1. Hexagonal Close Packing

2. Cubic Close Packing

3. Fluorite

Fluorite (body diagonal)

Fluorite (alternate)

When you have built these structures and have examined them, ask your instructor to approve your work before going on in this experiment.

Part II. From the following list, select 3 different structures (each may include structure alternates) and build them. After you have built these structures, ask your instructor to approve them before going on in this experiment.

a. Diamond b. NaCl (body diagonal)

NaCl (fcc) c. Perovskite

Perovskite (alternate) d. YBa2Cu3O7

YBa2Cu3O7 (alternate)

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e. Zinc Blende (body diagonal)

Zinc Blende (fcc)

Zinc Blende (expanded fcc)

Part III. Additionally examine several structures using Xtaldraw (on the PC) and/or models provided for you. You may find it interesting to look at some of the structures that you have already built, comparing and contrasting the different views of the structures simultaneously. Other suggestions included the following: Bravais Lattices, CO2, sulfur, C60 and C74 (compare stability), mica, and iron. Alternatively, explore some crystal structures that interest you!

A great reference: Mathur, N.; Thomas, R.M. “Power to Perovskite.” New Scientist

1998, 30-33.

186 Honors Chemistry Laboratory

“Everything You Need To Know About Crystals:

A Basic Introduction”

By: You!!!

As a final experience to your spring quarter crystal investigation, you will be authoring a guidebook covering topics necessary for understanding crystal basics. Imagine you have been appointed as the instructor for a one day short course entitled “Everything You Need to

Know About Crystals: A Basic Introduction”. Your guidebook will serve as the text for your course.

Here are some basic guidelines to follow:

• Topics covered should include those you consider to be essential to explaining

the basics of crystals, their structure, formation, behavior, etc…

• Your guidebook should be no more than 10 pages long.

• You should include illustrations – this is a very visual topic!

187 • You can consult your textbook, JCE software used in Honors Lab, your

instructors, credible web sources, and any other crystal experts you find to

compile your information – however be sure not to plagiarize.

• Be creative! You may organize your guidebook in any manner you see fit.

Further questions may be directed to your instructors – Good luck!

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