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Olesik Deparfinent of Chemistry UMX Number: 9822291 UMI Microform 9822291 Copyright 1998, by UMI Company. All rights reserved. This microform edition is protected against unauthorized copying under Title 17, United States Code. UMI 300 North Zeeb Road Ann Arbor, MI 48103 ABSTRACT A carbon fiber microelectrode has been used to develop and investigate a novel method for the titration of metal ions and other electroactive species. Electrochemical shielding of the center fiber from the analyte, by the formation of titrant at the outer pair, allows a stoichiometric endpoint to be determined. A complexometric titration using amperometric endpoint detection is therefore possible using this type of electrode arrangement. As a test case, electrogenerated EDTA was used to titrate BP* solutions of a few to several hundred micromolar concentration. The central fiber of the array acted as the indicator (collector) electrode for uncomplexed BP* by having its potential fixed sufficiently negative for reduction of BP* but not for reduction of the BiEDTA complex. By controlling the reduction current applied to the generator fibers, the quantity of titrant is controlled precisely. An increase in the generator current liberates more titrant into solution where it shielded the collector from the analyte. The generator current which resulted in maximum shielding of the collector was shown to be proportional to the bulk concentration of analyte. Because caiton-fiber microband arrays have been difficult to construct, they have not been frequently used. A simple, inexpensive, and relatively quick Ü procedure to construct these arrays has been developed and described. The electrodes are sufficiently reliable and rugged for repeated use and polishing, unlike electrode arrays formed by thin-film or vapor deposition techniques. The qualitative relationships of the electrode and experimental variables to the analytical results are explored. The quality and characteristics of the electrode array are shown to affect the quality and characteristics of the analysis. In addition, the nature of the caiton surfaces requires that modified procedures be used to achieve adequate electrode response for solution concentrations less than approximately 20 jaM. It is shown that the analysis is quick (endpoints typically reached in less than a minute), precise (1 to 5% RSD), and reliable. Ill I dedicate this work to my mother and family, who were always there to give support, and especially to the memory of my late father who was not able to see it completed. IV ACKNOWLEDGMENTS I wish to thank my advisor, Larry Anderson, for the Ideas and discussions which made the work In this thesis possible. I also thank Beth Groeber, Larry Gal, Husantha Jayaratna, and Hong Ren for their help with the problems which were too difficult to work out alone. This research was supported by grants from the Lubrizol Co. and the U.S. Department of Education. VITA October 18,1965 ................................................ Bom- Kenosha, Wisconsin 1988 ....................................................................... B.S. Chemistry, University of Illinois, Urbana, Illinois 1988 - 1991 .......................................................... Control Scientist, Burroughs-Wellcome Co., Greenville, North Carolina 1991 -1997.......................................................... Graduate Teaching Associate and Research Fellow, The Ohio State University 1997 - present ..................................................... Research Scientist, Wyeth-Ayerst Research, Pearl River, New York FIELDS OF STUDY Major Field: Chemistry VI TABLE OF CONTENTS Page Abstract.................................................................................................................. ii Dedication ................................................................................................................ iv Acknowledgements ................................................................................................ v Vita............................................................................................................................vi List of T ables ...........................................................................................................ix List of Figures ..........................................................................................................xi Chapters: 1. Introduction .......................................................................................................... 1 2. Experimental ....................................................................................................... 7 2.1 . R eagents .............................................................................................. 7 2.2. Electrode Fabrication ......................................................................... 8 2.2.1. Plating of Carbon Fibers .................................................... 8 2.2.2. Coating With Photoresist .................................................. 10 2.2.3 Fiber Arrangement and Immobilization ......................... .11 2.2.4. Electrical Connections ....................................................... 14 2.2.5. Fiber Polishing ....................................................................15 2.2.6. Electrode Characterization................................................15 2.2.7. Comments On Electrode Fabrication and U se .............19 2.3. Apparatus ............................................................................................22 2.3.1. Instrumentation ...................................................................22 2.3.2. Electrochemical Cell.......................................................... 25 2.4. Experimental Procedures ..................................................................2 7 2.4.1 . Preliminary cyclic voltammograms of metal ions 27 2.4.2. Electrochemical shielding experiments ..........................29 Vll 3. Theory...................................................................................................................35 3.1. EDTA as a complexometric titrant ...................................................35 3.2. EDTA titrations using electrochemical endpoint detection 39 3.3. Electrochemical Shielding at triple-band microfiber arrays .42 3.4. Mass transport at the electrode surface .........................................50 4. Results and Discussion ......................................................................................55 4.1. Reduction of metal ions and metal ion/EDTA complexes at carbon fibers ............................................................................................ 55 4.1.1. Determination of reduction potentials .............................56 4.1.2. Reduction overpotential and surface activity .................58 4.1.3. Mechanical instability of deposits .................................... 60 4.2. Swept-current experiments with BMBiEDTA ..............................61 4.2.1. High concentrations (greater than 20 pM B f^ ............. 62 4.2.2. The relationship of experimental variables to analytical characteristics............................................................... 67 4.2.3. Predeposition to increase electrode activity ..................78 4.2.4. Analysis of low concentration Bi^* solutions (less than 20 pM B P ).................................................................... .84 4.3. Stepped-current experiments ........................................................... 89 4.3.1. Analysis results .................................................................. 90 5. Conclusions ..........................................................................................................95