Abstract an Automated, On-Line Electrochemical

ABSTRACT

AN AUTOMATED, ON-LINE ELECTROCHEMICAL CHLORITE ION SENSOR

by John Nicholas Myers

An automated, on-line electrochemical chlorite ion sensor is reported. A three electrode assembly modified with sol-gel solid state electrolyte enabled electrochemical measurements in electrolyte-free liquids. Flow injection analysis was coupled with the electrode assembly in an electrochemical flow cell. The optimized parameters of the flow cell were a flow rate of 0.8 mL/min and an applied potential of ~0.5V. No faradaic interference was observed from perchlorate or chlorate ions. The concentrations of two validation standards as found by the method and ion chromatography were not statistically significantly different. An on-line, electrochemical chlorine dioxide sensor is also reported that couples gas-diffusion flow injection analysis with the developed electrochemical flow cell. The optimized parameters of the flow system and detector were a flow rate of 0.75 mL/min and an applied potential of -0.05V. The detection limit was .08 mgL-1 and 10 mg/L solutions of chlorite, chlorate, and perchlorate ions showed no interference.

AN AUTOMATED, ON-LINE ELECTROCHEMICAL CHLORITE ION SENSOR

A Thesis

Submitted to the

Faculty of Miami University

in partial fulfillment of

the requirements for the degree of

Master of Science

Department of Chemistry and Biochemistry

John Nicholas Myers

Miami University

Oxford, OH

2011

Advisor______Dr. Gilbert E. Pacey Reader______Dr. James. A. Cox Reader______Dr. C. Scott Hartley Reader______Dr. Richard T. Taylor

TABLE OF CONTENTS

Cover Page Abstract Table of Contents ii List of Tables iv List of Figures v 1. Introduction 1 1.A Water Disinfection with Oxy-Chlorine Chemicals 1 1.A.1 Introduction 1 1.A.2 Chlorine Chemistry in Aqueous Solutions 1 1.A.3 Chlorine Dioxide Chemistry 1 1.A.4 Chlorite Ion Electrochemistry 2 1.B The Sol-Gel Process 4 1.B.1 Introduction 4 1.B.2 Sol-Gel Chemistry and Processing 4 1.B.3 Sol-Gel Deposition and Templating Methods 5 1.B.3.a Evaporation-Induced Self-Assembly 5 1.B.3.b Solid-State Electrolyte 5 1.B.3.c Electrochemically Assisted Self-Assembly 6 1.C Flow Injection Analysis 6 1.C.1 Introduction 6 1.C.2 Gas-Diffusion FIA 7 1.C.3 Dispersion 7 1.C.4 Electroanalytical Flow Analysis 8 1.C.4.a Electroanalytical Flow Measurements 9 1.C.4.b Electrochemical Flow Cells 9 2. An Automated, On-Line Electrochemical Chlorite Ion Sensor 10 2.A Research Objectives 10 2.B Introduction 10 - 2.B.1 Current EPA Standard Methods for ClO2 Ion Monitoring 11 - 2.B.2 Electroanalytical Methods for ClO2 Ion Monitoring 12 2.C Experimental 13 2.C.1 Reagents 13 2.C.2 Probe Fabrication 13 2.C.3 Apparatus 14 2.D Results and Discussion 16 2.E Conclusions 26 3. An Automated, On-Line Electrochemical Chlorine Dioxide Sensor 27 3.A Introduction 27 3.A.1 E PA 4500-ClO2 D and Electroanalytical Methods 27 for ClO2 Monitoring 3.B Experimental 28 3.B.1 Reagents 28 3.B.2 Chlorine Dioxide Standardization and Handling 28 3.B.3 Apparatus 28

3.C Results and Discussion 30 3.D Conclusions 33 4. References 34

iii

LIST OF TABLES

Table 1: Redox reactions and potentials of the oxy-chlorine species. Adapted from reference 1. Table 2: Analytical Methods for Chlorine Dioxide and Related Compounds. Adapted from reference (1). Table 3: Comparison of validation standard concentrations determined with ion chromatography and the developed method. ū ± σ (n=3). Xm, mean; σ, standard deviation; sm, standard deviation of the mean

LIST OF FIGURES

Figure 1: TEOS precursor A) hydrolysis and B) condensation polymerization. Figure 2: Flow injection analysis configuration. Figure 3: Gas-diffusion cell for GD-FIA. Figure 4: FIA A) injection value and B) sample plug injection. Figure 5: The laminal flow profile of an injected plug. Figure 6: A) Thin-layer and B) wall-jet electrochemical flow cell configurations. Figure 7: Electrochemical probe electrode assembly. A, sol-gel solid-state electrolyte film; B, plastic mold (black) with epoxy filler (gray); R, Pt/Pto QRE; C, Pt CE; W, Pt WE Figure 8: Schematic diagram of FIA system with an EC probe flow cell detector. - Figure 9: Cyclic voltammogram of 50 mM ClO2 ion on a platinized platinum disk electrode. Scan rate 50 mV/S; pH 10.2. Figure 10: Cyclic voltammogram of 44.1 mM ClO2- ion at an EC probe; scan rate, 100 mV/s, pH 10.1. Figure 11: Scan rate studies at an EC probe (A) and corresponding peak current vs. scan rate1/2 (B). Curves (A) illustrate voltammograms recorded in deionized water in - the presence of 44 mg/L ClO2 ion. - Figure 12: Flow injection amperometric responses of an EC probe detector to 1.0 mg/L ClO2 ion injections. Applied potential, .40 V. - Figure 13: Flow injection amperometric responses of an EC probe to three 2.1 mg/L ClO2 injections and their %RSD vs. flow rate. Applied potential, .2 V. - Figure 14: Hydrodynamic voltammogram of 2.5 mg/L ClO2 at an EC probe detector. Figure 15: Flow injection amperometry of ClO2- (A) and corresponding calibration curve - (n=5) (B) at an EC probe detector. ClO2 concentrations 5, 2.5, 1.25, .63, .31 mg/L; applied potential, .50 V - Figure 16: Hydrodynamic voltammogram of 14.8 µM ClO2 ion (blue), 14.8 µM KNO3 - - (purple), 12.0 µM ClO3 ion (green), and 10.0 µM ClO4 ion (red) at an EC probe detector. Figure 17: Cyclic voltammograms of 44.1 mM ClO2- ion at three separately fabricated EC probes; scan rate, 100 mV/s, pH 10.1. Figure 18: Flow injection amperometric calibration curves of ClO2- at an EC probe detector. - ClO2 concentrations 2.0, 1.0, 0.8, 0.4, 0.2 mg/L; applied potential, 0.50 V. Figure 19: Schematic diagram of GD-FIA system with a Teflon membrane and an EC probe flow cell detector.

Figure 20: Flow injection amperometric responses of an EC probe to three 3.8 mg/L ClO2 injections and their %RSD vs. flow rate. Applied potential, 0 V. Figure 21: Hydrodynamic voltammogram of 2.5 mg/L ClO2- using GD-FIA with an EC probe detector. Figure 22: Flow injection amperometry of ClO2 using GD-FIA (A) and corresponding calibration curve (n=3) (B) at an EC probe detector. ClO2 concentrations, 1.6, 1.1, 0.5, 0.35 mg/L; applied potential, -0.05V.

1. WATER DISINFECTION WITH OXY-CHLORINE SPECIES

1.A.1 Introduction The main purpose of water disinfection is to limit waterborne disease and inactive pathogenic organisms. Disinfectants tend to be strong oxidants or generate oxidants as by-products1. Chlorine is the most widely used disinfection agent in United States public drinking water treatment because it is a very effective biocide; however, the process results in chlorinated organic disinfection by-products (DBP) such as trihalomethanes (THM) and haloacetic acids (HAA)1. Since these DBPs are potentially carcinogenic and mutagenic, the EPA, as directed by the Safe Drinking Water Act (SDWA)2, has established maximum contaminant levels (MCL) in drinking water. The MCL represents the concentration of these species in drinking water that will not cause adverse health effects. The DBPs produced through water chlorination and the ensuing EPA regulations have led to an increased interest in alternative disinfectants other than chlorine. The most popular alternatives are listed in the EPA Alternative Disinfectants and Oxidants Guidance Manual1: Ozone, chlorine dioxide, potassium permanganate, chloramine, ozone/hydrogen peroxide combinations, ultraviolet radiation. Intensive research has been conducted with these disinfectants in the last thirty to forty years to address the trade-off between pathogen inactivation and the potential health effects of disinfection by-products.

1.A.2 Chlorine Chemistry in Aqueous Solutions When chlorine gas is added to water, it rapidly hydrolyzes3 to form hypochlorous acid (HOCl): + - -4 Cl2(g)+ H2O ⇌ HOCl + H + Cl KH = 4 x 10 (25 °C) HOCl, being a weak acid, exhibits a pH dependent dissociation: - HClO ⇌ H+ + ClO pKa = 7.54 (25 °C)

Therefore, between pH 6.5-8.5, the typical pH of most drinking water, HOCl and OCl- are in - 4 equilibrium. The three species Cl2(g), HOCl, and ClO are called free available chlorine (FAC) . The germicidal efficiency of HOCl is higher than that of ClO-, so chlorination for disinfection is typically performed at lower pH1.

1.A.3 Chlorine Dioxide Chemistry The focus of the research was the use of chlorine dioxide for water system disinfection. One of the pressing concerns about chlorine dioxide is the formation of chlorite ion. Therefore, the measurement of chlorite ion is required; however, the only accepted method is ion chromatography (IC). The issue with IC is that it does not lend itself to in-line measurements. The goal of this research is develop a new method that meets the in-line requirements. Before any analytical method can be developed, it is imperative that we understand the chemistry of the chemical species. Chlorine dioxide (ClO2) is a popular alternative to chlorine for water treatment because it 4 disinfects over a wide pH range without chlorinating organic molecules . ClO2 is an energetic oxidizing agent that has a high solubility in dilute aqueous solutions where it exists as a free radical5 and a volatile dissolved gas. Oxidation primarily occurs through a selective one-electron 1 - transfer mechanism where ClO2 is reduced to chlorite ion (ClO2 ).

- - 0 ClO2(aq) + e ⇌ ClO2 E = 0.954V ClO2 also oxidizes many inorganic species, such as manganese(II) and iron(II), to form precipitates. As a germicide, ClO2 is typically equal to or superior to chlorine on a mole-dose basis1 and is used by approximately 700 to 900 United States public water systems to treat potable water. Chlorine dioxide must be generated at the point of use because it undergoes explosive 1 decomposition if its concentration exceeds 10% by volume in air . ClO2 is generated by oxidizing chlorite ion with gaseous chlorine Cl2(g), hypochlorous acid (HOCl), or hydrochloric acid1,4.

2NaClO2 + Cl2(g) ⇌ 2ClO2(g) + 2NaCl 2NaClO2 + HOCl ⇌ 2ClO2(g) + NaCl + NaOH 5NaClO2 + 4HCl ⇌ 4ClO2(g) + 5NaCl + 2H2O 2HClO2 + HOCl ⇌ 2ClO2 + HCl + H2O Depending on the reaction conditions, the reactions above can produce the unstable intermediate 4 - dimer {Cl2O2} or {Cl ClO2} - - - Cl2 + ClO2 ⇌ {Cl ClO2} + Cl which can react with free chlorine to produce chlorate ion, as described by the overall reactions. - - - + ClO2 + HOCl ⇌ ClO3 + Cl + H - - - + ClO2 + Cl2 + H2O ⇌ ClO3 + 2Cl + H

Chlorite ion can occur downstream from the chlorine dioxide generation if these generation reactions are incomplete or non-stoichiometric addition of the sodium chlorite and chlorine reactants were used.

1.A.3.a Chlorine Dioxide Disinfection Byproducts When ClO2 is used under typical conditions for water disinfection, approximately 50- 70% gets converted to chlorite ion and 30% to chlorate ion1. Organic DBPs are produced when - 1 ClO2 reacts with biological material. ClO2 ion is stable in the presence of the organic DBPs , but can be oxidized to chlorate ion under basic conditions in the presence of free chlorine secondary disinfectant. - - - - ClO2 + OCl ⇌ ClO3 + Cl

Richardson et. al6 found that although there are more than 40 different semi-volatile, organic DBPs produced from ClO2 disinfection, their concentrations were approximately 1-10 ng/L. The total organic halide concentrations (TOX) found was 0.10 and 0.05 mg/L with and without residual chlorine secondary disinfection, respectively.

1.A.4 Chlorite Ion Electrochemistry - The reduction potential of the ClO2 /ClO2 couple is the lowest in the oxy-chlorine species redox series (Table 1).

Table 1: Redox reactions and potentials of the oxy-chlorine species.

- 7,8,9 The ClO2 /ClO2 redox couple has been shown to be reversible on platinized platinum . This is quite unusual because most other simple aqueous anions do not form stable products upon one-electron oxidation. In 1967, Schwarzer and Landsberg7 reported that the oxidation of chlorite ion on a graphite rotating disk electrode in the pH range 5-9 produced a current that was proportional to concentration. Raspi and Pergola8 later investigated the reversibility of the redox couple using a platinized-platinum microelectrode. They reported a log[i(id – i)] vs. E plot showing a straight line with a slope of about 60 mV, which supports a one-electron transfer reaction. The most thorough investigation of the electrochemistry of chlorite ion was performed by Sinkaset and Trogler9 using rotating disc and ac voltammetry to measure the heterogeneous rate constant (kel). The reported cyclic voltammogram of chlorite had a ΔEp of ~70 mV and a ip,a / ip,c of ~1. The kel of the reaction was 0.014 ± 0.003 cm/s on a reduced platinum electrode with a ΔG‡ of 25 ± 3.3 kJ/mol. The heterogeneous rate constant was not appreciably affected by proton concentration when the pH was above 5. The kinetic experiments suggested that the heterogeneous electron transfer of chlorite ion oxidation is non-adiabatic. Based on the reported electrochemistry, we hypothesize that an electrochemical system could be used to measure chlorite ion in the presence of other oxy-chlorine species. The challenge is to overcome the electrolyte issues observed in prior methods. Given that a constant electrolyte concentration provides for more reproducible signals, many electrochemical measurements are performed at a near constant or constant electrolyte concentration. The implication for an in-line sensor system for drinking water is that the addition of electrolytes to the sample is not practical. The new electrochemical method must be designed to minimize the electrolyte effects. The potential answer is the use of sol-gel based electrodes.

1.B THE SOL-GEL PROCESS 1.B.1 Introduction The sol-gel process is a wet chemical technique used to make three-dimensional inorganic macromolecular networks10. The versatility of the process enables precise control of microstructure and bulk shape, resulting in solid materials with well defined composition, morphology, and porosity. In addition, organic components can be incorporated into the network to create organic-inorganic hybrids that combine the rigid three-dimensional inorganic architecture with easily accessible active sites in the pores of the material11. Tailoring the chemical, mechanical, optical, and electrical properties of these materials has found numerous applications in analytical chemistry, such as chemical and biological sensors, stationary phases for chromatography, anti-reflective coatings, and electrode modifiers10.

1.B.2 Sol-Gel Chemistry and Processing A colloid system contains particles ranging in size from ~1-1000 nm that are distributed in a medium12. The medium is called the dispersion medium or the continuous phase and the particles are called colloidal particles or the dispersed phase13. In this size range, there are typically 103 to 109 atoms per particle. Colloidal particles, due to their small size, do not settle out of the dispersion medium like typical particles in a suspension because short-range forces between colloidal particles and molecules of the dispersion medium, such as van der Waals attraction and surface charges, overcome the gravitational force and prevent sedimentation12. The small diameters of colloidal particles result in high surface area. High surface area contributes to pronounced surface effects, such as adsorption on the colloidal particles or the electrical double layer that forms at the interface between colloidal particles and dispersion mediums of different phases. Sol-gel processing involves a sol, a colloidal suspension of solid particles in a liquid or gas dispersion medium, and a gel, which is an interconnected solid network of macroscopic dimensions that expands in a liquid medium from the polycondensation of monomer precursors that can make more than two bonds12,13. The continuous solid network of gels also contains a continuous liquid phase due to mechanical enclosure of the liquid phase and internal adsorption sites. The monomer precursors used to create the colloidal particles in the sol are typically metals or metalloids surrounded by organic alkyl or alkoxy ligands. Silica sol-gels use silicon alkoxide precursors such as tetraethyl orthosilicate (TEOS), which is one of the most common and thoroughly studied precursors in sol-gel processing. TEOS precursors readily hydrolyze in water once the pH of the near-neutral sol is changed to an appropriately acidic or basic value12: Si(OEt)4 + H2O → HO-Si(OEt)3 + EtOH The partially hydrolyzed products can then undergo condensation polymerization: (OEt)3Si-OH + HO-Si(OEt)3 → (OEt)3Si-O-Si(OEt)3 + H2O (OEt)3Si-OEt + HO-Si(OEt)3 → (OEt)3Si-O-Si(OEt)3 + EtOH TEOS can form four bonds by replacing each ethoxy ligand, resulting in complex three- dimensional branching of the polymer (Figure 1). If the monomers are allowed to condense until the average polymer size is at least several nanometers, a sol is formed. The process of successive polymerization, gelation, aging, and drying of the sol results in a gel. The experimental parameters used to form the gel control the resulting microstructure and porosity.

Figure 1: TEOS precursor A) Hydrolysis and B) condensation polymerization.

1.B. 3 Sol-Gel Thin Film Deposition and Templating Methods Interest in sol-gel processing of supported thin films has grown considerably in recent years due to numerous applications of patterned, mesoporous nanostructured materials in optical, electronic, electrochemical and sensing devices10. One of the key developments that allowed this research to flourish was sol-gel deposition using surfactant template-based evaporation-induced self-assembly (EISA)14. In template-based sol-gel processing, gelation occurs around a template that has been doped into a sol. Subsequent removal of the template leaves holes in the solid matrix of the same shape, size, and orientation of the template15. An interesting application of this technique was developed by Widera and Cox where a silica sol-gel thin film templated with nanochannels and doped with electrolyte served as a solid-state electrolyte for electrochemical measurements. One of the main limitations of the EISA technique is the difficulty in controlling pore orientation. A recently developed technique, electrochemically assisted self-assembly, addresses this problem by using surfactant self-assembly under electric potential control as a template for cylindrical pores oriented normal to a conducting support.

1.B.3.a Evaporation-Induced Self-Assembly First pioneered by Mobil Oil Corp researchers in 1992, EISA uses surfactant self- assembly in ethanol/water solutions of soluble silica species to co-assemble silica-surfactant mesophases14,16. After sol-gel processing by dip or spin coating, the surfactant concentration, initially under its critical micelle concentration, increases as ethanol evaporates (which consequently prevents further acid catalyzed polycondensation of the colloidal particles). As the surfactant concentration increases (Figure 2) silica-surfactant micelles self-assemble which undergo further organization into liquid-crystalline mesophases. EISA enabled unprecedented control of the final mesostructure and porosity obtained using silica sol-gel processing, however the technique was not able to easily form pores oriented normal to the support, which limited accessibility from the film surface16. In addition to this, flat surfaces are normally required for dip and spin coating utilizing EISA, and the process covers the entire surface, rather than selected areas, with a thin film.

1.B.3.b Solid-State Electrolyte Widera and Cox demonstrated that electrochemical measurements can be taken in highly resistive media by embedding a three-electrode assembly in solid-state sol-gel electrolyte17. The sol-gel was prepared with a surfactant template and electrolyte doped into the precursor solution. Nanochannels within the sol-gel thin film served as ionic bridges that allowed ionic current to flow between electrodes. Electrochemical reactions occur at the three-phase boundary (3PB) of

the electrochemical probe (EC probe) between the analyte solution, the electrode, and the conductive sol-gel. It was hypothesized that the negative silanyl groups on the silica backbone minimized mass transport of ions from the pores of the sol-gel into contacting liquid phases, thus providing ionic conductivity within the sol-gel.

1.B.3.c Electrochemically Assisted Self-Assembly In 2007, Walcarius developed an electrochemical method to deposit homogenous, mesoporous silica thin-films that contained nanopores oriented perpendicular to the plane of a conductive support called 'electrochemically assisted self-assembly' (EASA)16. In this method, an electrode is immersed in a hydrolysed sol solution that contains a surfactant template. The application of a positive potential to the electrode produces a positive electric field that induces self-assembly of the surfactant into stable aggregates at the solid-liquid interface, regardless of the surfactant concentration is below the critical micelle concentration. Surfactant phases that compose hemisphere/sphere micelles and hemicylinder/cylinder micelles can then used as templates to produce mesoporous materials. If the potential is sufficient to generate hydroxyl ions, polycondensation of the precursors occurs around the surfactant template, producing a mesoporous silica thin-film with hexagonally packed silica nanochannels normal to the electrode surface. Cyclic voltammetry demonstrated that a redox probe was able to reach the electrode surface, which suggests that the pores remained open at the both the film-solution and electrode- film interfaces

1.C. FLOW INJECTION ANALYSIS 1.C.1 Introduction Flow Injection Analysis (FIA) is an automated, unsegmented, continuous flow analytical method where chemical measurements are taken within a flowing liquid. First introduced by Ruzicka and Hansen18 in 1975, FIA was developed to modernize and automate the sample- handling step of chemical analysis, which has typically been performed using batch methodology with volumetric glassware19. In traditional FIA, samples are injected as narrow plugs into a nonreactive continuous carrier stream that combines with a separate reagent stream in a mixing coil, resulting in a detectable species that flows through a downstream detector (Figure 2). The signal generated by the detector is an asymmetric transient peak with a height that is proportional to analyte concentration.

Sample plug Carrier stream

pump coi

detector Reagent steam Figure 2: Flow injection analysis configuration.

Automated, on-line handling of sample and reagents is achieved through highly reproducible operational sequences with defined timing20. Reproducible timing assures that samples and standards undergo the same analytical process and reaction conditions, which

enables precise, non-steady state measurements to be performed. Separation techniques such as solvent extraction, dialysis, and gas diffusion are thus easily automated with FIA because, unlike with the manual operation, there is no need for complete analyte recovery given the high precision of the separation.

1.C.2 Gas-Diffusion Flow Injection Analysis (GD-FIA) In GD-FIA, a hydrophobic, microporous, gas permeable membrane is used to separate a liquid phase donor stream containing a gaseous analyte from an acceptor stream that contains reagent and leads to a detector (Figure 3)20. The partial pressure difference across the membrane causes dissolved volatile species in the sample to diffuse from the donor stream into the acceptor stream at a rate dependent on the gas volatility and the permeability of the membrane21, which provides discrimination between different gases; nongaseous materials remain in the donor stream, which eliminates many interferents and thus greatly increases the selectivity of the technique

Acceptor To stream detector

Waste

Figure 3: Gas-diffusion cell for GD-FIA.

1.C.3 Dispersion Standard FIA systems use an injection valve to reproducibly inject a sample as a plug by flushing a loaded sample loop with the carrier stream, creating a rectangular sample zone (Figure 4)

Figure 4: FIA A) injection valve and B) sample plug injection.

As the plug is transported downstream, the rectangular profile changes. A laminar flow profile develops19 (Figure 5) because liquid moving near the tubing has to overcome friction, which reduces its velocity.

Figure 5: The laminal flow profile of an injected plug.

The sample plug also broadens due to the concentration gradient between the carrier liquid and the sample, resulting in longitudinal diffusion parallel to the column and radial diffusion perpendicular to the flow direction. The dilution experienced by the plug is quantified by defining dispersion, D19,20:

D = (1) 0 퐶 퐶 where C0 is the concentration in the original injection and C is the peak concentration at the detector. Dispersion can easily be controlled by changing the sample volume, tubing length, and flow rate. Manipulating these FIA parameters therefore leads to controllable sample dispersion19 and hence highly reproducible mixing between sample and reagent occurs.

1.C.4 Electroanalytical Flow Analysis FIA can be coupled with electroanalytical methods by using electrochemical flow cells as detectors to measure electrochemical properties of target analytes22. Flow-through

electrochemical detectors primarily measure current using fixed-potential amperometry, where the potential of a working electrode is constant and current is measured as a function of time. The main benefits of flow-through EC detectors are low detection limits, high selectivity towards electro active species, low dead volumes, fast response time, inexpensive instrumentation, and easy miniaturization23. Many of these detectors are commercially available for routine applications in clinical, environmental, and industrial laboratories that require high throughput, automated analysis.

1.C.4.a Electroanalytical Flow Measurements Performing electrochemical measurements under hydrodynamic conditions increases the mass transport rate to the electrode surface, shortens the diffusion layer, and continually washes away electrode reaction products22. In addition, there is no double layer charging when hydrodynamic fixed-potential amperometric measurements are taken, which significantly reduces the background24. The current measured by a FT EC detector is governed by the rate of electron transfer reactions occurring at the WE surface. If the heterogeneous charge transfer kinetics of interest are fast, the current will be mass-transport-limited. The mass-transport-limited current of an analyte is proportional to the peak concentration of the plug (c) according to24: Ilim = kmc (2) where km is the mass transport coefficient: = (3) The Nernst approximate method푛퐹퐴퐷22 describes the inverse relationship between diffusion 푚 layer thickness and flow rate, U: 푘 훿 = (4) 퐵 Where B and α are constants based on the훼 hydrodynamic conditions. The mass-transport limited current of flow-through electrochemical훿 푈 detectors is therefore proportional to : 훼 Ilim = (5) 훼 푈 푛퐹퐴푘푚푐푈 1.C.4.b Electrochemical Flow Cells퐵 Electrochemical flow cell detector performance is measured using the standard operational parameters sensitivity, selectivity, noise, limit of detection, precision, and accuracy24. The sensitivity and selectively are controlled by adjusting the potential. Large potentials will increase the current; however, lower potentials result in more selectivity. In addition to requiring high signal to noise ratios and low dead volumes, electrochemical flow cells need well-defined hydrodynamics and a small ohmic drop between the working and counter electrode22. The counter and reference electrodes are usually placed downstream of the working electrode to prevent interference from reaction products at the counter electrode or leakage from the reference electrode. Low dead volume, which minimizes band broadening, is achieved by placing the working electrode close to the column. The two most common amperometric detector flow cells are the thin-layer and wall-jet (Figure 6) configurations22,23.

Inlet Outlet Inle t

Outlet Outlet

WE WE

Figure 6: A) Thin-layer and B) wall-jet electrochemical flow cell configurations.

In the thin-layer configuration, the stream flows through a thin rectangular channel that contains an embedded planar electrode, resulting in a very small dead volume. In the wall-jet configuration, the stream flows directly against an embedded planar electrode (wall). One of the main drawbacks of routine flow electroanalysis with fixed-potential detection is that noble metal electrodes often require cleaning and regeneration due to surface fouling caused by adsorption of reactants or reaction products, which makes it difficult to maintain a constant electrode surface. Pulsed amperometric detection (PAD) mitigates the electrode surface fouling problem by applying an anodic cleaning pulse and a cathodic reactivation pulse after anodic detection22. Another problem encountered is that added supporting electrolyte is normally required to lower the carrier stream resistance, maintain a constant ionic strength, and eliminate electro-migration effects24.

2. AN AUTOMATED, ON-LINE ELECTROCHEMICAL CHLORITE ION SENSOR 2.A Research Objectives - These EPA standard methods for the daily monitoring of ClO2 and ClO2 ion are time consuming, require a skilled operator, consume reagents, are not readily portable, and are not readily developed into on-line continuous monitoring systems. The main goal of this research was to address these problems by developing automated, on-line sensors that could daily monitor these species in ClO2-disinfected water in accordance with EPA regulations. The research objectives are: 1) develop a new chlorite ion sensor system that can be automated and placed in the line water line containing chlorine dioxide disinfected water and 2) develop a chlorine dioxide sensor system that can be used in the presence of the new chlorite ion sensor system. Both sensor systems must be comparable in analytical performance to the existing method. It is not the objective of this thesis to provide sufficient data to allow EPA to judge the equivalency between the current standard methods and the new sensor systems.

2.B Introduction Chlorine dioxide is a popular alternative to chlorine for water treatment because it 4 disinfects over wide pH ranges without chlorinating organic molecules . When ClO2 is used

under typical conditions for water disinfection, approximately 50-70% gets converted to chlorite ion and 30% to chlorate ion1. The United States EPA has regulated a 1 mg/L maximum - 26 contaminant level (MCL) for ClO2 ion in drinking water because it can potentially cause hemolytic anemia and adverse nervous system effects. In addition to this, EPA regulations state that facilities that use chlorine dioxide for water disinfection must monitor daily chlorite ion and chlorine dioxide concentrations in the water. The EPA 40 CFR §141.131(b) states that chlorite ion must be monitored with Amperometric Titration Standard Method 4500-ClO2 E (Table 2) or 1 Ion Chromatography EPA Method 300.1 ; the EPA 40 CFR §141.131(c) states that ClO2 must be monitored using DPD Standard Method 4500- ClO2 D or Amperometric Method II Standard Method 4500- ClO2 E (Table 2). The EPA standard methods for monitoring chlorite ion are expensive, time-consuming, require batch analysis by a skilled operator, and are not readily incorporated into automated, in- line monitoring systems. These problems make compliance with EPA chlorite ion regulations - burdensome. A possible solution is to develop an automated, in-line ClO2 ion sensor. Automated analysis would simplify the monitoring process by obviating manual sample collection, sample preparation, and batch analysis20. - Electrochemical sensors are promising candidates for monitoring ClO2 ion given their low cost, fast response times, ease of miniaturization, and stability25. In addition, electrochemical detection is relatively easy to automate due to recent advances in flow-through electrochemical detectors for FIA and capillary electrophoresis23. The in-line electrolytic measurement of oxy-chlorine species in drinking water; however, is difficult due to low electrolyte concentrations in disinfected water. Even the approved amperometric method has some difficulties with the measurement of chlorite ion pure drinking water. - This research describes an electrochemical ClO2 ion sensor that couples FIA automation with in-line electrochemical detection. An electrochemical flow cell is described that integrates an 3PB electrode into a FIA flow system for in-line, amperometric detection in high resistance - drinking water without added electrolyte. The main hypothesis is that the ClO2 ion concentration in drinking water can be selectively measured with an amperometric sensors based - on the prior sol-gel work. Given that ClO2 ion has the lowest oxidation potential of all the oxy- chlorine species, it will oxidize at a lower potential than the other potentially interfering oxy- - chlorine species present in a ClO2 disinfected drinking water matrix. The primary goal is to develop an alternative method to the EPA standard methods for chlorite ion monitoring by incorporating the sensor into an in-line, continuous monitoring system.

- 2.B. 1 Current EPA Standard Methods for ClO2 Ion Monitoring 1 In the EPA Alternative Disinfectants and Oxidants Guidance Manual , the current ClO2 Analytical Methods are compared (Table 2)

Table 2: Analytical Methods for Chlorine Dioxide and Related Compounds. Adapted from reference 1.

These analytical methods are thoroughly described and compared in the 1987 AWWA 4 Disinfectant Residual Measurement Techniques . The amperometric method 4500-ClO2 E is an - amperometric titration using phenylarsine oxide. The method can distinguish ClO2, Cl2, ClO2 , and ClO-, but the accuracy and precision of the method are largely based on the analytical skill of the operator. Ion chromatography, Method 300.1, requires bulky, expensive instrumentation in addition to a trained laboratory technician. - 2.B.2 Electroanalytical Methods for ClO2 Ion Monitoring Casella27 recently reported the use of silver nano islands grown on a glassy carbon electrode to electroreduce chlorite that showed resistance to surface poisoning, but no detection limits were given and the response times were high (6-8s). In previous work 28, a tungsten oxide - electrode was used as an amperometric sensor for ClO2 with a detection limit of 0.4 mM. Ohura and Yamasaki29 developed a potentiometric FIA technique that was able to simultaneously - - determine ClO2 ion and ClO3 . A potential change was observed an a redox electrode based on - the oxy-chlorine species reactions with a Fe(III)-Fe(II) buffer, however ClO2 ion interfered with - 30 the measurement if its concentration was higher than that of ClO3 . Pezzatini and Innocenti - used pulse voltammetry with a carbon electrode to measure the oxidation of ClO2 to ClO2. The results were not significantly different from those found with ion chromatography; however, the electrode required extensive polishing and electrochemical pretreatment prior to measurements. This flaw eliminates these electrodes from consideration as an in-line sensor.

2.C Experimental 2.C..1 Reagents The precursor to the silica sol–gel was tetraethyl orthosilicate (TEOS), 99.999% purity, obtained from Sigma Aldrich Chemical Company (Milwaukee, WI). House-distilled water, which was deionized with a Millipore Milli-Q Advantage A10 system, and Spectrophotometric Grade ethanol (Aldrich) were used as co-solvents in the preparation of the silica. The surfactant template used in the preparation of the silica was Triton X-114 (Sigma Chemical Co., St Louis, MO). Ammonium hexafluorophosphate, >95% purity (Aldrich), was doped as an electrolyte into the silica during gelation. The analyte was ultra-pure sodium chlorite (produced in Dr. Gordon’s laboratory, Miami University). The platinum electrode materials were purchased from Sigma. All glassware was chemically pretreated prior to use to remove chlorine demand. All glassware was washed with soap and water followed by a 24 hour soak in ~1 v/v % HNO3. After this, the glassware was soaked in dilute Clorox bleach for at least 24 hours and removed just prior to use.

2.C.2 Probe Fabrication Figure 7 shows the cell design of the EC probe. Platinum wires (0.2 mm dia.) composed the working (WE), counter (CE), and quasi-reference electrodes (QRE).

WE CE QRE

Figure 7: Electrochemical probe electrode assembly. A, sol-gel solid-state electrolyte film; B, plastic mold (black) with epoxy filler (gray); R, Pt/Pto QRE; C, Pt CE; W, Pt WE.

The electrodes were supported in a 0.6cm outer diameter (OD), 0.4 cm inner diameter (ID) plastic tube filled with an epoxy resin made by mixing EPO-TEK 353ND Part A and Part B (Epoxy Technology, Inc) in a 10:1 ratio by weight. The epoxy was cured for 30 minutes at 80°C, after which the surface was smoothed with 400 grit sandpaper. The counter and reference elec-

trodes were coplanar with the surface of the epoxy, whereas the working electrode protruded above this plane. The three electrodes were electrochemically pretreated prior to sol-gel deposition by scanning the potential from -1.0 to 1.0 V vs. Ag/AgCl at .1 V/s in .1M H2SO4. The plane was then coated with a silica sol–gel solid-electrolyte film, which was cast from 5 uL of sol. The silica sol-gel consisted of 0.1 mL of TEOS, 9.0 mL of ethanol, 1.0 mL of water, 0.6 mL of water saturated with NH4PF6, and 0.2 mL of Triton X-114. The sol was magnetically stirred for 2 hours prior to use for film formation. The gelation time was seven days under ambient laboratory conditions. A portion of the working electrode and all surfaces of the reference and counter electrodes were coated with silica sol-gel using this procedure. All electrochemical measurements were made with a model 650C electrochemical workstation from CH Instruments (Austin, TX). All potentials were measured and reported vs a Pt/PtO quasi-reference electrode unless otherwise indicated.

2.C.3 Apparatus A schematic of the flow injection analysis system is shown in figure 8. The flow system consisted of a syringe pump connected to a Rheodyne 7125 6-port syringe loading injection valve with a 250 uL external sampling loop. The detector used was an electrochemical flow cell produced by inserting the protruding portion of the EC probe's working electrode directly into the downstream flow from the injector. The carrier stream was deionized water. The flow rate of the carrier stream was 0.8 mL/min unless otherwise indicated. The flow system used 0.5-mm-i.d. Tygon tubing. Fixed-potential amperometric i-t curves were recorded using this FIA arrangement and the resultant peak heights were measured using GRAMS/32AI software.

Figure 8: Schematic diagram of FIA system with an EC probe flow cell detector.

2.D Results and Discussion The electrochemical behavior of chlorite ion at a platinum electrode and an EC probe was studied using cyclic voltammetry. The cyclic voltammogram of chlorite ion at a platinized Pt disk macroelectrode (Figure 9) had a peak separation of 0.098V with an E0 of 0.75 vs. Ag/AgCl; the anodic to cathodic peak current ratio was 1.04. These characteristics support the quasi- - 7,8,9 reversible behavior of the ClO2 / ClO2 redox couple reported in literature . The slight deviation from ideal behavior in the peak separation has been hypothesized to be due to slow heterogeneous charge transfer kinetics9. 50 mM chlorate and perchlorate ions could not be distinguished from the deionized water background.

- Figure 9: Cyclic voltammogram of 50mM ClO2 at a platinized platinum disk electrode. Scan rate 50 mV/S; pH 10.2.

The cyclic voltammogram of chlorite ion at an EC probe (Figure 10) had a peak separation of ~0.25V, a gradually increasing oxidation current, and a reduction peak. The additional peak separation compared to the platinized Pt electrode is likely due to uncompensated resistance in the sol-gel electrolyte. The gradually increasing oxidation current is hypothesized to be due to a hindrance of chlorite ion mass transfer to the electrode surface caused by the electrostatic repulsion of the negatively charged ion from the negatively charged silanol groups on the sol-gel. The peak for the reduction of chlorine dioxide to chlorite ion supports this because chlorine dioxide is not charged. It is possible that surface fouling of the Pt also contributed to the distortion of the oxidation peak9. 10.0 mg/L chlorate and perchlorate ions could not be distinguished from the deionized water background.

Figure 10: Cyclic voltammogram of 44.1 mM ClO2- ion at an EC probe; scan rate, 100 mV/s, pH 10.1

- The diffusion of ClO2 ion to the EC probe surface was analyzed by plotting the oxidation peak current at 0.2V from cyclic voltammograms vs. the square root of scan rate (Figure 11). This plot displayed a straight line, which suggests that the oxidation of chlorite ion at an EC probe is mass-transfer limited.

Figure 11: Scan rate studies at an EC probe (A) and corresponding peak current vs. scan rate1/2 (B). Curves (A) illustrate voltammograms recorded in deionized water in the presence of - 44.1 mg/L ClO2 ion.

The EC probe flow cell detector performance was tested by measuring its response to - repeated injections of ClO2 ion using flow injection amperometry. Asymmetric peaks (Figure 12) were observed that rose sharply and decayed slowly with a baseline-to-baseline time of approximately three minutes. The asymmetric peaks were representative of injected plugs that have undergone dispersion, which suggests that the dead volume between the tubing and the 3PB is very small.

- Figure 12: Flow injection amperometric responses of an EC probe detector to 1.0 mg/L ClO2 ion injections. Applied potential, 0.40 V.

Optimization of the EC probe flow cell detector was achieved by measuring how the flow rate and applied potential affected the signal. Changes in flow rate had a significant effect on the hydrodynamic conditions at the 3PB due to numerous possible flow paths within the flow cell. The signal slightly fluctuated with changes in the flow rate; however the %RSD of the measurements ranged from ~0.75 to 4 (Figure 13). Therefore, a flow rate of 0.8 mL/min was chosen due a minimum in %RSD.

Figure 13: Flow injection amperometric responses of an EC probe detector to three 2.1 mg/L - ClO2 injections and their %RSD vs. flow rate. Applied potential, 0.2 V.

The optimized applied potential was found using hydrodynamic voltammetry at the optimized flow rate. The beginning of the limiting-current plateau region of the hydrodynamic voltammogram (Figure 14), ~0.5V, was used as the applied potential. Higher potentials were not investigated because they resulted in increases in background current likely arising from the oxidation of water. If the pH at the WE surface were to dramatically decrease (pH<~4), acid- catalyzed sol-gel processing would compromise the stability of the 3PB

- Figure 14: Hydrodynamic voltammogram of 2.5 mg/L ClO2 at an EC probe detector.

The sensitivity of the optimized FIA system with an EC probe detector for the determination of chlorite ion in the low mg/L range was evaluated by recording calibration curves of peak height vs. concentration over the range of 0.3 to 5 mg/L (Figure 15). Linear least squares fit of the data (5 points) yielded the following: slope, 6.86 nA / mgL-1; y-intercept, -0.55 ; and R2, 0.99. The noise was obtained by taking the standard deviation of the intercept when all points were plotted to account for sample introduction flicker noise caused by the poorly defined hydrodynamic conditions. The detection limit (S/N = 3) was 0.08 mg/L.

Figure 15: Flow injection amperometry of ClO2- (A) and corresponding calibration curve - (n=5) (B) at an EC probe detector. ClO2 concentrations 5.00, 2.50, 1.25, 0.63, 0.31 mg/L; applied potential, 0.50 V.

The selectivity of the sensor was evaluated by monitoring the optimized detector response to solutions of chlorate and perchlorate ions. The response of 1.0 mg/L solutions showed small oxidation peaks, which was unexpected because no oxidation current was observed in the cyclic voltammogram of the solutions at an EC probe at 0.5V. To investigate this, a hydrodynamic - voltammogram (Figure 16) was plotted with 14.8 µM ClO2 ion (1 mg/L), 14.8 µM KNO3, 12.0 - - µM ClO3 ion (1 mg/L) and 10.0 µM ClO4 ion (1 mg/L) from 0.3-0.45V.

- Figure 16: Hydrodynamic voltammogram (A) of 14.8 µM ClO2 ion (blue), 14.8 µM KNO3 - - (purple), 12.0 µM ClO3 ion (green), and 10.0 µM ClO4 ion (red) at an EC probe detector and - the figure without ClO2 ion (B).

- As the potential increased, the ClO2 ion peak height increased at a much higher rate than that of - - KNO3, ClO3 ion, or ClO4 ion. The peak heights for KNO3 were similar in magnitude to those of - - ClO3 ion and ClO4 , which suggests that the signal is non-faradaic. This effect is likely due to charging of the double layer at the 3PB due to the higher conductivity of the plug compared to the deionized water carrier stream. The effect would not be a problem for the sensor system, since the charging would build up and level off giving a constant slightly elevated baseline. The accuracy of the developed method was evaluated by measuring the concentrations of two spiked validation standards (A and B) and then comparing the results with ion chromatography (IC) (Table 3). The concentrations of the validation standards were not significantly different from the accepted values and the values found using the proposed method and ion chromatography were not significantly different.

Table 3: Comparison of validation standard concentrations determined with ion chromatography and the developed method. ū ± σ (n=3). Xm, mean; σ, standard deviation; sm, standard deviation of the mean.

Cyclic voltammetry was used to test the reproducibility of the EC probe fabrication process. Three reduction peaks were observed at ~0, 0.05, and 0.08V and one clear oxidation peak was at ~0.25V (Figure 17). These small changes in potential can primarily be attributed to the variable potential of the QRE based on the particular Pt/PtO ratio. The current differences are likely due to different active 3PB areas.

Figure 17: Cyclic voltammograms of 44.1 mM ClO2- ion at three separately fabricated EC probes; scan rate, 100 mV/s, pH 10.1.

The long term stability of the sensor was tested by plotting calibration curves on various days after probe fabrication (Figure 18) when the sensor was constantly flushed with a low flow rate of deionized water. The slope fluctuated between 9.55 - 1.29E-08 A / mgL-1 over the course of about three weeks and the detection limits ranged from 0.02 - 0.13 mgL-1 (Table 4)

Figure 18: Flow injection amperometric calibration curves of ClO2- at an EC probe detector. - ClO2 concentrations 2.0, 1.0, 0.8, 0.4, 0.2 mg/L; applied potential, 0.50 V.

Slope Intercept DL Day ( A/mgL-1) (A) R2 (mgL-1) 1 1.13E-08 4.55E-10 0.99 0.02 2 1.07E-08 -8.12E-11 0.99 0.04 5 9.70E-09 -6.06E-10 0.99 0.12 9 9.55E-09 1.65E-10 0.99 0.04 10 1.19E-08 -3.72E-10 0.99 0.08

19 1.29E -08 3.24E -11 0.96 0.13

Table 4: Parameters for calibration curves measured on days one, two, five, nine, ten, and nineteen after EC probe fabrication.

2.E Conclusions A sensor system for the automatic, in-line, selective determination of chlorite ion is reported. Electroanalytical measurements were performed in electrolyte-free liquids by using an EC probe, which enables in-line detection in high-resistance media such as disinfected water. Cyclic voltammetry scan rate studies suggest that the current arising from the oxidation of chlorite ion at an EC probe is mass-transfer limited. By coupling FIA with an EC probe electrochemical flow cell, automated analysis was achieved. The optimized parameters of the flow system and detector were a flow rate of 0.8 mL/min and an applied potential at the limiting- current of the hydrodynamic voltammogram, which is ~0.5V. This sensor is intended to fulfill the daily monitoring requirements of the EPA DBP regulations for chlorite ion. Detection limits of 0.02-0.13 mg/L were attained, which is about one order of magnitude below the MRDL. The sensor showed no faradaic signal for perchlorate 26

or chlorate ions, which demonstrates the high selectivity of the measurement. The concentrations of spiked validation standards as found by the proposed method and ion chromatography were not statistically different, which supports the accuracy of the method. The lifetime and stability of the sensor were investigated by measuring calibration curves over time under constant-flow conditions. Detection limits of <0.1 mg/L were repeatedly achieved over a period of three weeks.

3 AN AUTOMATED, ON-LINE ELECTROCHEMICAL CHLORINE DIOXIDE SENSOR

3.A Introduction Chlorine dioxide is a popular alternative to chlorine for water treatment because it disinfects over wide pH ranges without chlorinating organic molecules4. The United States EPA has regulated a 0.8 mg/L maximum residual disinfectant level (MRDL) for ClO2 in drinking 26 - water to insure that ClO2 ion concentrations do not exceed their MCL. The EPA standard methods for monitoring chlorine dioxide are expensive, time-consuming, require batch analysis by skilled operator, and are not readily incorporated into automated, in-line monitoring systems. These problems make compliance with EPA chlorine dioxide MDL regulations burdensome. Again, electrochemical sensors are a potential solution to these problems. However, the biggest issue with chlorine dioxide methods has been that they also measure several of the oxy-chlorine species that would be present in the water. Many methods actually convert the oxy-chlorine species (particularly chlorite ion) into chlorine dioxide and measure chlorine dioxide before and after the conversion. Given that chlorine dioxide is usually in high concentration compared to the oxy-chlorine species, these determinations by difference have been problematic. This research describes an electrochemical ClO2 sensor that couples the selective method of GD-FIA with the EC probe electrochemical flow cell detector previously described. The main hypothesis is that the ClO2 concentration in drinking water can be selectively measured with an amperometric detector after separation from the oxy-chlorine species. The primary goal is to develop an alternative method to the EPA standard methods for ClO2 monitoring by incorporating the sensor into an on-line, continuous monitoring system.

3.A.1 EPA 4500- ClO2 D and Electroanalytical Methods for ClO2 Monitoring The DPD Standard Method 4500- ClO2 D is a colorimetric method where ClO2 reacts with the dye N-N’-diethyl-p-phenylenediamine (DPD). The chlorine dioxide reacts with the DPD resulting in a decrease in absorbance that is proportional to the ClO2 concentration. The method requires glycine as a masking agent for chlorine and the potential interferences are monochloramine, oxidized manganese, chlorite ion, and chromate ion. There have only been a few reports of the electrochemical detection of ClO2. A 31 voltammetric rotating Teflon membrane electrode was used to measure about .30 mg/L ClO2 and showed no interference from hypochlorite, chlorite, chlorate, or permanganate ions; however, the 90% response times was about one minute. Glassy carbon and platinum electrodes 32 have been used for the amperometric detection of ClO2 for pulp bleaching , however experiments were only performed at pH 4 and no detailed parameters were provided.

3.B Experimental 3.B.1 Reagents Chlorine dioxide was produced based on a previously published method33 where chlorite ion is oxidized by persulfate ion according to equation:

2- 2- 2- 2 ClO + S2 O8  2 ClO2 + 2SO4

Chlorine dioxide stock solutions were refrigerated below 6°C in the dark without headspace to prevent evaporation and decomposition.

3.B.2 Chlorine Dioxide Standardization and Handling Dilute solution of chlorine dioxide (<10-3 M) were analyzed spectrophotometrically at 259 nm utilizing a molar absorptivity value of 1250 M-1cm-1. Samples were placed in capped 1 cm quartz cells and all absorbance measurements were performed using an Olis-Cary 14 spectrophotometer with a high resolution (up to 0.1 nm) monochromator. Chlorine dioxide stock solutions were diluted and handled within an aluminum foil-wrapped 30 mL shrinking bottle34.

3.B.3 Apparatus A schematic of the GD-FIA system is shown in Figure 19. The flow system consisted of a donor stream supplied by a syringe pump connected to a Rheodyne 7125 6-port syringe loading injection valve with a 250 uL external sampling loop and a separate acceptor stream both connected to a Tecator Chemifold V gas-diffusion manifold in a countercurrent arrangement35. The detector was the EC probe flow cell previously described. The membrane used was a strip of ACE Teflon (PTFE) tape. The flow rates of the donor and acceptor streams were 0.75 mL/min of deionized water unless otherwise indicated. The flow system used 0.5-mm-i.d. Tygon tubing. Fixed-potential amperometric i-t curves were recorded with negative values assigned as reduction current using this FIA system and the resultant peak heights were measured using GRAMS/31AI software.

Figure 19: Schematic diagram of GD-FIA system with a Teflon membrane and an EC probe flow cell detector.

3.C Results Optimized parameters for GD-FIA were previously investigated by Canham35 and Hollowell36. The best performance for a non-reacting species was achieved when the acceptor and donor had equal flow rates that were as low as possible in a countercurrent flow configuration. However, as discovered earlier, flow rate had a large effect on the hydrodynamic conditions within the EC probe flow through detector. To investigate this, the reduction current signal (ip,c) was plotted vs. flow rate (Figure 20). The lowest %RSD occurred at 0.75 mL/min and was thus used. Slower flow rates did result in higher signals, but caused a high noise background that resulted in a much higher %RSD.

Figure 20: Flow injection amperometric responses of an EC probe to three 3.8 mg/L ClO2 injections and their %RSD vs. flow rate. Applied potential, 0 V.

The optimized applied potential was found by plotting a hydrodynamic voltammogram at a low flow rate (Figure 21). The beginning of the limiting-current plateau region of the hydrodynamic voltammogram, -0.05V, was used as the applied potential. Lower potentials were not investigated because they resulted in increases in background current arising from the reduction of hydronium and water. If the pH at the WE surface were to dramatically increase, base-catalyzed sol-gel processing would compromise the stability of the 3PB.

Figure 21: Hydrodynamic voltammogram of 3.8 mg/L ClO2 using GD-FIA with an EC probe detector. Flow rate, 0.45 mL/min.

The sensitivity of the optimized GD-FIA system with an EC probe detector for the determination of chlorine dioxide in the low mg/L range was evaluated by recording calibration curves of peak height vs. concentration over the range of 0.3 to 1.6 mg/L (Figure 22). Linear least squares fit of the data (5 points) yielded the following: slope, 5.40 nA / mgL-1; y-intercept, - 0.25 ; and R2, 0.99, . The noise was obtained by taking the standard deviation of the intercept. The detection limit (S/N = 3) was 0.08 mg/L

Figure 22: Flow injection amperometry of ClO2 using GD-FIA (A) and corresponding calibration curve (n=3) (B) at an EC probe detector. ClO2 concentrations, 1.6, 1.1, 0.5,0.35 mg/L; applied potential, -0.05V.

Injections of 10 mg/L solutions of chlorite, chlorate, and perchlorate ions could not be distinguished from the baseline, which supports the selectivity of the measurement.

3.D Conclusions A method for the automatic, selective determination of chlorine dioxide is reported. GD- FIA was used to increase the selectivity of the method because ClO2 exists as a dissolved gas in solution. Electroanalytical measurements were performed in electrolyte-free liquids by using the EC probe electrochemical flow cell. The optimized parameters of the flow system and detector were a flow rate of 0.75 mL/min and an applied potential at the limiting-current of the hydrodynamic voltammogram, ~-0.05V. This sensor is intended to fulfill the daily monitoring requirements of the EPA DBP regulations for chlorite ion. A detection limit of 0.08 mg/L was observed, which is one order of magnitude below the 0.8 MRDL. The sensor showed no signal for 10 mg/L solutions of perchlorate, chlorate, or chlorite ions which demonstrates the high selectivity of the measurement. The two sensor system reported in this thesis could be combined in a split stream automated system to simultaneously measure chlorite ion and chlorine dioxide. No tests were performed to verify this approach.

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