Chapter 25. Voltammetry

Home , Amperometric titration, Cyclic voltammetry, Hydrodynamic voltammetry, Polarography, Voltammetry

z Excitation Signal in Voltammetry z Voltammetric Instrumentation z Hydrodynamic Voltammetry z Cyclic Voltammetry z Pulse Voltammetry z High-Frequency and High-Speed Voltammetry z Application of Voltammetry z Stripping Methods z Voltammetry with Microelectrodes

Voltammetry z Voltammetry: measurement of current (I) as a function of applied potential (E). Under condition with polarization (η). Negligible consumption of analyte – Amperometry: measure I at a fixed E – Potentiometry: measure E when I Æ 0, no polarization – Coulometry: measure C, polarization is compensated, all analyte is consumed z Polarography: voltammetry at the dropping mercury electrode (DME) – DA: Hg (poison), apparatus (cumbersome), better techniques z Application: – Oxidation and reduction process – Adsorption processes on surfaces – Electron transfer mechanism Jaroslav Heyrovsky 1890-1967

1 Excitation Signals and Instrumentation

z WE: E (relative to RE); RE: constant E; CE: Pt wire (current) z Supporting electrolyte: a salt added in excess to the analyte solution, like alkali metal salt Measure I, I-to-E converter – No reaction at the E E follower, high Z, no I region – Reduce effect of migration E I – Lower R of the E i o E = E o solution o i R Eo = -IiR

An op amp potentiostat

Voltammetric Working Electrode z Disk electrode: A small flat disk in a rod of an inert materials like Teflon, glass or Kel-F. z HMDE: hanging mercury drop electrode – Large negative E, fresh metallic HMDE surface, reversible reaction z UME: microelectrode, r: < 25 µm, wire Disk electrode in glass, tip polished WE z Flow cell WE: in flowing stream, PEEK (polyethertherketon) z Emin: reduction of water (H2), Emax: oxidation of water (O2)

UME Flow electrode

2 Modified Electrode

z Chemical modification: – Irreversibly adsorbing substances: z oxidation of electrode (metal or C) surface (O- or –OH) z electrodeposition – Covalent bonding of components : z like SAM of thiols with amine or carboxyl group on the other end z Organosilanes or amines – Coating of polymer films z Dip coating, spin coating z Application: – Electrocatalysis – Smart window: electrode changes color upon reaction – Analytical sensor

Circuit Model of a Working Electrode

Double layer A. Randles circuit: Diffusion Bulk electrolyte layer – RΩ, solution resistance – Cd, double layer capacity WE – Zf, faradaic impedance Æ f dependence B. Faradaic impedance:

– Rs, electron transfer resistance – Cs, pseudocapacitance, mass transfer A Cd C. Faradaic impedance: RΩ – Rct, charge transfer resistance Zf – Zw, Warburg impedance B C Cd Cd

RΩ RΩ Zw Rs Cs Rct

3 Concentration Profile in Unstirred Solution

A planar electrode with potential step z Reaction: A + e- Æ P reversible and rapid z Mass transfer: 1. Migration: electric field; Supporting electrolyte (100×) 2.Diffusion: concentration gradient 3.Convection: mechanical z Potential vs. surface concentration: 0 0 0.0592 cP Eappl = E − log − Eref A n 0 cA A z Current:

∂c i = nFAD A ∂x n: #electron P F: Faraday constant A: surface area, cm2 D: diffusion coefficient, cm2/s Concentration distance profile during diffusion controlled reaction

Hydrodynamic Voltammetry

Flow pattern in a flow stream z the analyte solution is kept in continuous motion – stir the 0 0 0.0592 cP solution, Eappl = E − log − Eref A n 0 – flow cA solution, like in HPLC Flow patter near an electrode

10 ~ 100 µm

c0 = c − c0 convection P A A

A + e- Æ P reversible and rapid

4 Voltammograms z Voltammetric wave: an ∫-shaped wave of I-E z Limiting current, il: the current plateau observed at the top, ∝ cA il = kcA Linear-sweep voltammogram at slow scan rate – cA = 0 at electrode surface – maximum mass transfer rate z Current in American way: E0 = -0.26 V – Reduction current + – Oxidation current - z Half-wave potential: 0 – E1/2 at i = il/2, ≠ E – Relative to E0 – Identification

vs. SCE

Volumetric Currents

z A planar electrode: Nernst diffusion layer δ control n : electron / analyte ∂cA nFADA 0 F : 96485C/mol electron A + ne → P i = nFADA( ) = (cA − c ) A : electrode surface area, cm2 A 2 ∂x δ DA : diffusion coefficient, cm /s 3 0 cA : mol / cm z Limiting current: cA at the electrode surface =δ :0.Nernst diffuion layer thickness, cm nFADA il = cA = kAcA δ z Reverse current: cP in the bulk solution = 0. nFADP 0 nFADP 0 0 i = (cP − cP ) = cP = kPcP δ δ z Half-wave potential, E1/2: i = il/2

0 0.0592 kA 0.0592 i Eappl = EA − log − log − Eref n kP n il − i 0 0.0592 kA 0 E1/ 2 = EA − log − Eref ≈ EA − Eref n kP

5 Voltammetric I-E

z Based on the kinetics of the reaction: – Reversible systems: obey Nernst ∆E = 0.1 V equation ∆E = 0.2 V – Totally irreversible system: either the cathodic or anodic reaction is too slow as to be negligible – Partially reversible system: the reaction in one direction is much slower than the other one. – like organic system, i = kc, E = f(v, c, il) z Voltammogram for mixture: – ∆E ≥ 0.1 V z Anodic/Cathodic Voltammogram: – A: oxidation current – – B: both reaction – C: reduction current +

Clark electrode Oxygen Wave and Sensors z Oxygen wave: – I is proportional to n – Sparging: deaerate the solution with inert gas, N2, Ne and He – Highly depends on the pH of the solution z Clark electrode: volumetric sensor + – Cathodic Pt electrode: O2 + 4H + 4e ↔ 2H2O – Anodic Ag electrode: Ag + Cl- ↔ AgCl (s) + e – Diffusion across membrane ( ~ 10 µm) – Diffusion cross the thin electrolyte solution ( ~ 10 µm) – Steady-state current Æ I is dependent on electrochemical equilibrium, [O2] Æ 10 ~ 20 s and dm+s < 20 µm

6 Enzyme-based Sensors

• Glucose detection: largest selling chemical instruments • A polycarbonates film (glucose permeable, not for protein and other blood constitutes): diffuse through • An immobilized enzyme layer (glucose oxidase): glucose reduction Æ H2O2 • A cellulose membrane layer for H2O2 diffusion: H2O2 oxidation Æ O2 – Amperometric detection (I ∝ c) or volumetric detection (E ∝ c) of sucrose, lactose, ethanol and L-Lactate

glucose oxidase glu cose + O2 ⎯⎯→⎯⎯⎯⎯⎯ gluconic acid + H2O2 − − H2O2 + 2OH → O2 + H2O + 2e

Amperometric Titration z At least one species is Analyte is reduced electrochemical active z A WE (rotating Pt) + RE: confined to product either a precipitate or a stable complex. + - 2+ 2- – Ag for X , Pb for SO4 - – Exception: Br2 (BrO3 ) titration of organics produced is reduced Both analyte z Two WEs: and products are reduced – simple instrument, determination of a single specie – Karl fisher titration for determining water

− − + BrO3 + 5Br + 6H → 3Br2 + 3H2O

7 Rotating Electrodes

O2 reduction

z Rotating electrode: – RDE: rotating disk electrode, affiliate mass transfer – RRDE: rotating ring disk electrode, intermediate detection – Levich equation: 1/ 2 −1/ 6 il = 0.620nFADω v c

n : electron / analyte D : diffusion coefficient, cm2/s ω : angular velocity, radians/s v : kinematic viscosity, cm2/s 3 cA : mol / cm

RDE RRDE

The ripples are caused by the constant forming and dropping of Polarography the mercury electrode

z WE: DME, diffusion control, no convection z Residue current: current observed in the absence of an electroactive specie Polarogram z Diffusion current: limiting current which is limited by the diffusion z A: DL ~ 10-5 M, Faster equilibrium + new electrode surface Æ reproducible current; 0.5 mM Cd2+ in 1 M HCl High η for H2 evolution Æ low E window z DA: new surface Æ large charging current

1/ 2 2 / 3 1/ 6 (id )max = 708nD m t c 1 M HCl n : electron / analyte D : diffusion coefficient, cm2/s m : rate of flow of Hg throug the capillary, mg/s 3 cA : mol / cm t : time, s

8 Cyclic Voltammetry z CV: forward scan, switching potential, reverse scan z Application of CV: 3- 6.0 mM Fe(CN)6 – Study of redox reaction – Detection of reaction Reversible intermediates (reduction) – Observation of follow-up reactions (+) (-) z Reaction: (oxidation)

– A: H2O oxidation Æ O2 – B-H: reduction 0 Irreversible or rapid removal of Red – B-D: cA Æ 0 (reduction) 0 – D-F: cA = 0, δ↑ – F-H: reduction (+) (-) – H-K: oxidation E (vs SCE)

CV- Fundamental Studies z Peak potential: E and E 0.0592 pc pa ∆Ep = Epa − Epc = – Reversible: ∆Ep = 0.0592 /n n – Irreversible: ∆Ep > 0.0592 /n z Peak current: n : electron / analyte D : diffusion coefficient, cm2/s 5 3 / 2 1/ 2 1/ 2 A : electrode surface area, cm2 ip = 2.686 ×10 n AD v c c : mol / cm3 Parathion in 0.5 M v :scan rate, V/s acetate buffer in 50% z Qualitative information in organic and ethanol, pH = 5 inorganic chemistry – first choice – reaction intermediate

− + A:φNO2 + 4e + 4H → φNHOH + H2O B :φNHOH → φNO + 2H + + 2e− C :φNO + 2H + + 2e− → φNHOH

9 CV of Modified electrode

z Reversible surface redox couple Æ no mass transfer effect Æ symmetrical peaks + same peak height ∆Ep = Epa − Epc ≈ 0

Digital Simulation of CV

z Digital simulation: DigiSim, DigiElk – Fast implicit finite difference methods – 1st or 2nd order homogeneous chemical reaction – Generate dynamic concentration profiles – The exact current may be offset as the nonfaradaic current is not easily simulated

10 Differential Pulse Polarography

z DPP: increasing sensitivity – Lower DL: ~ 10-7 to 10-8 M (2 ~ 3 order lower than CV) ∆t – Enhancing faradic current: diffusion current (id) + Nernst contribution due to ∆E, several times larger than i , d 0.36 ppm ∆t is small enough tatrecylineHCl in 0.1 – Decrease in nonfaradic M acetate buffer, current: charging current pH=4 decays exponentially with time, is small at the late lifetime of the drop, ∆t is large enough – Trace heavy metal detection

Square-wave Polarography z SWP: increasing sensitivity – Great speed: step < 10 ms, signal average is 10 mV possible -7 -8 – Lower DL: ~ 10 to 10 M 50 mV = 2ESW – Enhancing faradic current + Decrease in nonfaradic current

– ∆I = If –Ir, the current difference is plotted

difference

Guanine, adenine, thymine forward

reverse

SWP generation

11 Stripping Methods z Stripping methods: – Anodic stripping methods: C Æ A for metal – Cathodic stripping methods: A Æ C for halides z Electrodeposition step: – Stirring the solution: mass transfer

– Only a fraction of analyte is Anodic stripping methods deposited: accumulation process – Depends on c, stir rate, deposition time, electrode surface and potential Cd – t < 1 min. for c ~ 10-7 M – t > 30 min. for c ~ 10-9 M, (higher sensitivity) – HMDE or noble metal (Pt, Au, Ag and C)

Microelectrodes

z Microelectrode: r ~ 1 to 20 µm – r >> δ, normal electrode, short time – δ >> r, UME, long time, steady state z Advantage: 50 µm – Small current (I ~ pA to nA) Æ small IR drop Æ no RE

– Capacitor charging current (Inf ∝ A) Æ Inf ↓ Æ faster scan – Faradaic current (If ∝ A/r) Æ bigger contribution from If Ælower DL – Rate of mass transport increases Æ steady state is established within µs Æ faster kinetic study, higher S/N ration 0 ⎛ 1 1 ⎞ i = nFADcA⎜ + ⎟,δ = πDt – Little disturbance to the system under ⎝ δ r ⎠ study – Small sample volume – Small current Æsystem with low dielectric constants, like toluene

12 Homework z 25-2 (a, b, c, e), 25-5