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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 by 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 ii 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 iv 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. v 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 5 radical and a volatile dissolved gas. Oxidation primarily occurs through a selective one-electron 1 - transfer mechanism where ClO2 is reduced to chlorite ion (ClO2 ). 1 - - 0 ClO2(aq) + e ⇌ ClO2 E = 0.954V ClO2 also oxidizes many inorganic species, such as manganese(II) and iron(II), to form precipitates.
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