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Stationary Electrode Voltammetry and Chronoamperometry in an Alkali Metal Carbonate-Borate Melt

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Stationary Electrode Voltammetry and Chronoamperometry in an Alkali Metal Carbonate-Borate Melt AN ABSTRACT OF THE THESIS OF DARRELL GEORGE PETCOFF for the Doctor of Philosophy (Name of student) (Degree) in Analytical Chemistry presented onC (O,/97 (Major) (Date) Title: STATIONARY ELECTRODE VOLTAMMETRY AND CHRONOAMPEROMETRY IN AN ALKALI METAL CARBONATE - BORATE. MFT T Abstract approved: Redacted for Privacy- Drir. reund The electrochemistry of the lithium-potassium-sodium carbonate-borate melt was explored by voltammetry and chrono- amperometry. In support of this, a controlled-potential polarograph and associated hardware was constructed.Several different types of reference electrodes were tried before choosing a porcelain mem- brane electrode containing a silver wire immersed in a silver sulfate melt.The special porcelain compounded was used also to construct a planar gold disk electrode.The theory of stationary electrode polarography was summarized and denormalized to provide an over- all view. A new approach to the theory of the cyclic background current was also advanced. A computer program was written to facilitate data processing.In addition to providing peak potentials, currents, and n-values, the program also resolves overlapping peaks and furnishes plots of both processed and unprocessed data. Rapid-scan voltammetry was employed to explore the electro- chemical behavior of Zn, Co, Fe, Tl, Sb, As, Ni, Sn, Cd, Te, Bi, Cr, Pb, Cu, and U in the carbonate-borate melt. Most substances gave reasonably well-defined peaks with characteristic peak potentials and n-values.Metal deposition was commonly accompanied by adsorp- tion prepeaks indicative of strong adsorption, and there was also evi- dence of a preceding chemical reaction for several elements, sug- gesting decomplexation before reduction. Chromium exhibited behav- ior indicative of a succeeding irreversible chemical reaction.Diffu- sion coefficients for copper and chromium were determined by chrono- 2/sec, amperometry.For Cu(II) to Cu(0), D = 1.94 ± .37 x106cm 2/sec. while for Cr(VI) to Cr(III), D = 2.14 ± .42 x10-6cm Stationary Electrode Voltammetry and Chronoamperometry in an Alkali Metal Carbonate-Borate Melt by Darrell George Petcoff A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy June 1970 APPROVED: Redacted for Privacy Professor .Ainalytical Chemistry in charge of major Redacted for Privacy Chairman of Department of Chemistry Redacted for Privacy Dean of Graduate School Date thesis is presented 1q70 Typed by Opal Grossnicklaus for Darrell George Petcoff ACKNOWLEDGEMENT The author is deeply indebted to Dr. Freund for maintaining an atmosphere amenable to both professional and personal growth.The computer programming associated with this work would not have been possible without the help of R. Jay Murray, and both he and John A. Starkovich are thanked for their assistance in this. A special debt of gratitude is owed to someone who wishes to remain anonymous for the donation of a high temperature furnace which enabled the development of the high temperature porcelain so useful in this study. The author also thanks the National Science Foundation for support in the form of a research grant and an NSF Cooperative Fellowship. A grant of computer time from the Oregon State University Computer Center is also gratefully acknowledged. TABLE OF CONTENTS I. INTRODUCTION 1 II. PROPERTIES OF THE CARBONATE MELT 5 The Lux-Flood Acid-Base Theory 5 Physical Properties of the Ternary Carbonate Eutectic 7 The Domain of Acidity in Molten Carbonates 8 Water and the Carbonate Melt 12 Containers for Molten Carbonates 14 III. ELECTROCHEMISTRY IN MOLTEN CARBONATES 16 The Anodic Electrolysis Reaction 16 The Cathodic Electrolysis Reaction 17 Standard Electrode Potentials in Fused Salts 20 Previous Electroanalytical Work Reported in the Literature 23 IV. REFERENCE ELECTRODES 27 Reference Electrodes in Fused Salts 27 The Oxygen Electrode in Fused Carbonates 29 The Danner-Rey Reference Electrode 31 V. THEORY AND PRACTICE OF ELECTROANALYTICAL METHODS 33 Conventional Polarography in Fused Salts 33 Stationary Electrode Polarography 35 Introduction 35 Reversible Charge Transfer 37 Introduction 37 The Initial' and Boundary Conditions 39 Solution and Results 41 Discus sion 44 Derivative Relationships 47 Theory of Irreversible Charge Transfer 52 Coupled Chemical Reactions 56 Introduction 56 The Diagnostic Criteria 56 A Chemical Reaction Preceding a Reversible Charge Transfer.Case III. 61 A Chemical Reaction Preceding an Irreversible Charge Transfer.Case IV. 63 Charge Transfer Followed by a Reversible Chemical Reaction.Case V. 64 Charge Transfer Followed by an Irreversible Chemical Reaction.Case VI. 65 Catalytic Reaction with Reversible Charge Transfer.Case VII. 67 Adsorption 68 Subsequent Theoretical Studies 70 Chronoamperometry 72 Theory 72 Unshielded Electrodes 73 VI. APPARATUS 75 High Temperature Equipment 75 Temperature Controller and Furnace 75 The Furnace Cell 82 Polarographic Equipment 87 The Periodic Shorting Microelectrode 87 The Controlled-Potential Polarograph 91 Theory of Operation 91 Details of Construction 95 Associated Hardware 99 The Sweep Generator 99 The Pulse Gate 99 Readout Devices 103 The Switching Circuit for Chronoamperometry 104 The Diode Cell Simulator 105 Operating Procedure 112 Balancing the Operational Amplifiers 112 Calibrating the Scan Voltage 113 Reference Electrodes 115 Early Work 115 The Clay Electrode 117 The Porcelain Membrane Electrode 118 Stability Tests 122 Polarization Tests 122 Microelectrodes 129 VII.RESULTS AND DISCUSSION 136 Theoretical Considerations 136 Denormalized Plots of the Eight Theoretical Cases 136 Theory of the Anodic Background Current 146 Computer Analysis of Data 151 Stoichiometry of the Ternary Carbonate-Boric Anhydride Reaction 153 Polarography 157 Early Studies 157 The Chloride Melt 157 The Carbonate Melt 160 Stationary Electrode Voltammetry 162 Introduction 162 Copper 164 Results 164 Discussion 167 Chromium 175 Results 175 Discussion 182 Additional Exploratory Work 185 Introduction 185 The Anomaly 186 Nickel 186 Cobalt 191 Iron 192 Uranium 194 Zinc 194 Cadmium 194 Lead 197 Tin 199 Bismuth 208 Arsenic 211 Antimony 214 Thallium 217 Tellurium 221 Chronoamperometry 225 VIII. SUMMARY 230 BIBLIOGRAPHY 234 APPENDIX I 248 APPENDIX II 254 LIST OF TABLES Table Page 1. Theoretical EMF series for molten carbonates at 600°C. 22 2. Results of a polarographic investigation in supporting electrolyte of fused carbonates by Delimarskii and Tumanova. 25 3. Chronopotentiometric determination of diffusion coefficients in fused carbonates by Tumanova. 26 4. Relationships for the determination of n-values from the regular and first derivative theoretical curves. 51 5. The eight cases for stationary electrode voltammetry. 57 6. Composition of a high temperature porcelain suitable for reference electrodes. 119 7. Reference electrode stability studies. 123 8. Stoichiometry of the ternary carbonate-boric anhydride reaction at 550°C. 155 9. PSM studies of niobium in LiCl-KC1 eutectic at 450°C.159 10. Copper n-values determined at a gold wire electrode. 168 11. Peak potentials and n-values for copper at a gold disk electrode. 169 12. Current relationships for copper at a gold disk electrode. 170 13. n-values for chromium (VI) at a gold wire electrode. 178 1/2 14. ip/v vs. v for the reduction of chromium (VI). 179 15. Potential and current data for chromium (VI) at a gold disk electrode. 180 Table Page 16. Voltammetric data for nickel (II) at a gold wire electrode. 190 17. Voltammetric data for zine. 195 18. Voltammetric data for cadmium. 196 19. Voltammetric data for lead (II) at a gold wire electrode. 200 20. Voltammetric data for tin at a gold wire electrode.I. 206 21. Voltammetric data for tin at a gold wire electrode.II.207 22. Voltammetric data for bismuth, 210 23. Voltammetric data for arsenic. 212 24. Voltammetric data for antimony. 218 25. Voltammetric data for thallium. 220 26. Voltammetric data for tellurium. 223 27. Diffusion coefficients for chromium and copper. 228 28. Cathodic peak potentials for various metals in molten alkali metal carbonate-borate at 550°C. 233 LIST OF FIGURES Figure Page 1. A. Potential-time waveform for cyclic voltammetry. B.Cyclic polarogram. 43 2. Log plot of log (i/N,7-i.) vs. E for the theoretical curve.46 3. Log plot of log (i/ip-i) vs. E for the theoretical curve. 48 4. Regular and derivative theoretical peaks. 50 5. Variation of the peak current functions with rate of voltage scan. 59 6. Ratio of anodic to cathodic peak currents as a function of rate of voltage scan. 60 7. Rate of shift of potential as a function of scan rate. 60 8. Temperature controller diagram. 77 9. Controller wiring diagram. 78 10. The polarographic cell. 83 11. The bubbler tube. 86 12. The coulometry cell. 88 13. The periodic shorting microelectrode. 89 14. The controlled-potential polarograph. 93 15. Wiring diagram of the controlled-potential polarograph.96 16. Operational amplifier manifold. 97 17. The pulse gate. 100 18. The diode cell simulator. 106 19. Simulator waveforms. 108 Figure Page 20. Correction curves for derivative peak shifts caused by noise filters. 110 21. A simulated peak polarogram and and its derivative. 111 22. Reference electrodes. 125 23. Reference electrode polarization tests. 126 24. Clay electrode micropolarization test. 127 25. Porcelain electrode micropolarization test. 128 26. Planar gold microelectrode. 131 27. A comparison of wire and planar gold microelectrodes.135 28. Case I.Reversible charge transfer. 137 29. Case II.Irreversible charge transfer. 138 30. Case III.Preceding chemical reaction. 139 31. Case IV.Preceding reaction.Irreversible charge transfer. 140 32. Case V.Succeeding reversible reaction. 141 33. Case VI.Succeeding irreversible reaction. 142 34. Case VII.Catalytic reaction.Reversible charge transfer. 143 35. Case VIII.Catalytic reaction.Irreversible charge transfer. 144 36. Theoretical anodic waves and backgrounds. 148 37. Theoretical anodic background curves. 149 38. Theoretical overlapping peaks. 154 Figure Pa e 39. Regular and PSM polarograms for .004 M niobium in LiCl-KC1 eutectic at 450°C. 158 40. Voltammograms for copper (II). 165 41. Single scan voltammograms for copper (II).
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