Electrochemistry, Past and Present - American Chemical Society

Chapter 26

Fortuitous Experiments Downloaded by UNIV OF CALIFORNIA SAN DIEGO on April 5, 2016 | http://pubs.acs.org Publication Date: January 1, 1989 | doi: 10.1021/bk-1989-0390.ch026

Discoveries of Underpotential and of Quantitative Anodic Stripping Voltammetry L. B. Rogers Department of Chemistry, University of Georgia, Athens, GA 30605 Both phenomena were encountered in studies aimed toward using platinum, gold, or graphite electrodes, rather than mercury, to isolate carrier-free radiotracers. Preliminary "polarographic" scans of millimolar silver ion using platinum electrodes followed by reversal of the scan to clean the deposit from the electrode produced large sharp peaks. A guick calculation indicated the promise of that approach for trace analyses. Then, electrodeposition of carrier-free radio-silver found fractional monolayers depositing much sooner on platinum than predicted by conventional use of the Nernst Eguation. Later, studies at M.I.T. suggested that structural parameters of the substrate could be related to the size of the underpotential - or its absence. The electrochemical experiments described below were performed during a 30-month period when the writer was a group leader in long-range research in analytical chemistry at Clinton Laboratories, now known as Oak Ridge National Laboratory. One major goal of the research group was to explore the use of electrodeposition as a means of isolating quantitatively carrierfree radionuclides. Another goal was to explore ways for determining trace amounts of elements; for example, non-radioactive nuclides of silver were also present in carrier-free preparations partly because they, too, were produced in the bombardment with neutrons and be0097-6156/89/0390-0396$06.00/0 © 1989 American Chemical Society

Stock and Orna; Electrochemistry, Past and Present ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

26. ROGERS

Underpotential and Quantitative Anodic Stripping Voltammetry 397

cause they probably were also present as an impurity in the target prior to the irradiation. A third goal developed later from our early success in continuously recording "polarographic" (voltammetric) data using solid electrodes. That small step in automatic recording, using a Sargent-Heyrovsky polarograph, made possible the rapid survey of many half-cell reactions, especially those run in a series in the same solution. Downloaded by UNIV OF CALIFORNIA SAN DIEGO on April 5, 2016 | http://pubs.acs.org Publication Date: January 1, 1989 | doi: 10.1021/bk-1989-0390.ch026

Background for the developments Voltammetry. Although Heyrovsky had developed polarography using a dropping mercury electrode in the early 1920s, it was in 1941 that Kolthoff and Lingane (1) forcefully brought the technique to the attention of American chemists. For elements soluble in mercury, a constant halfwave potential, independent of the initial concentration of the reducible ion, made identification relatively easy. That potential can be related to the standard halfcell potential by taking into account a shift in potential as a result of amalgam formation. Shortly thereafter, Laitinen and Kolthoff (2.) showed that the halfwave potential for a metal deposited upon a solid electrode changed with the initial concentration in accord with predictions based upon the Nernst Equation. Voltammograms were obtained manually by changing the potential in small increments and waiting 2-3 minutes after each voltage setting for a steady current to be reached. The deposit was dissolved, after removing the electrode from the solution, before another "polarogram" was run. Control of electrode potential. Historically, one adjusted the pH and allowed hydrogen evolution to limit the potential while the electrolysis ran at constant current. Hickling (3.) was the first to employ instrumental control of the potential. Most recently, Lingane (4) had reported a 3-electrode system for electromechanically controlling the electrode potential in both directions. Automatic control opened up the possibility of performing selective reductions over a wide range of potentials with only a minimum concern for pH. Radiation and radioactivity. Immediately following World War II, there was a great deal of interest in the isolation of fission products and of carrier-free nuclides produced by neutron bombardments followed by decays into other radioactive species. Although many chemists were using liquid-liquid extraction, ion exchange and, sometimes, copreciptation with a "foreign" carrier, no one at the laboratory was exploring electrodeposition. However, the writer did find a report (then classified as secret) by his predecessor, D. N.


398

ELECTROCHEMISTRY, PAST AND PRESENT

Hume, which stated that the electrochemical behavior of a dropping mercury electrode did not appear to be noticeably affected by a high neutron flux or the presence of a modest level of radioactivity. Presumably the same would be true for other electrode materials.

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on April 5, 2016 | http://pubs.acs.org Publication Date: January 1, 1989 | doi: 10.1021/bk-1989-0390.ch026

Experimental goals Basic idea. Substitution of a platinum electrode for a dropping mercury electrode appealed to the writer as a means of greatly reducing the problem of contamination. Further reflection suggested that, when a carrier-free radiotracer was deposited, the chances were heavily in favor of its forming only a fractional monolayer of deposit. Hence, the conditions would be very similar to those that resulted in a constant halfwave potential for mercury-soluble elements. The possibility of being able to deposit an element, regardless of its initial concentration in solution, at a preselected potential for a given electrode area and volume of solution, was very attractive. Overall plans. The initial strategy included several steps. First, it was desirable to confirm Laitinen's work using a platinum electrode for 10- 3 to 10- 5 M (close to the lowest measureable concentration) reducible ion. Second, a suitable element had to be selected, preferably one with a rapidly reversible oneelectron transfer, a relatively long halflife so as to minimize the need for frequent preparations of tracer, and a suitably high specific activity. A thorough search by A. F. Stehney led to selection of Ag-111 prepared by neutron bombardment of palladium. Although nearly 500 times more non-radioactive Ag-110 was produced at the same time, calculations showed that, barring the presence of significant amounts of silver impurity in the palladium (or other sources of contamination) , concentrations of 10-9 M or less should be sufficiently active for 2-3 weeks. Shortly thereafter, J. C. Griess (5) started work on an isolation procedure based upon dissolution of the palladium target followed by isolation of the radio-silver by electrodeposition from a cyanide solution. Extensions to the original plan. To speed the gathering of current-voltage "polarographic" data using a platinum electrode, we first explored the use of a Sargent Heyrovsky polarograph which changed the potential continuously and recorded the current on a drum holding photographic film (6) . Although the reduction curves for silver sometimes had maxima, the halfwave potentials usually gave data that agreed within 10 mv and



26. ROGERS Underpotential and Quantitative Anodic Stripping Voltammetry 399 also agreed closely with the values predicted from the Nernst Equation. Second, we modified the procedure for cleaning the deposit from the electrode between runs. The usual procedure of removing the platinum electrode from solution, dissolving the deposit in nitric acid, and reconditioning the electrode surface before running another voltammogram was frustrating because of the time it consumed. Out of desperation, the writer decided to run the potential scan in the opposite direction to clean off the deposit. A large, sharp peak was obtained over a relatively short range of potential. A quick calculation showed that the coulombs checked well with those involved in the deposition step. Furthermore, because of the sharpness of the peak, quantitation was easier, especially for concentrations less than 10- 5 M. Starting two years later at M.I.T., Lord and O'Neill (7.) employing faster scan rates, demonstrated quantitative anodic stripping voltammetry down to 10- 1 1 g of silver. Third, when concerns arose that the silver impurity in the platinum electrodes might distort the deposition data at the trace level, the writer substituted a pencil lead and, later, a graphite spectroscopic electrode in the voltammetric runs. A small ( 0.1 V or less) overvoltage was found in the first run with a new electrode, but later runs on the same electrode showed no overvoltage. Again, voltammetric behavior of graphite electrodes was later examined in more detail by Lord (8) at M. I. T. while demonstrating their utility for organic oxidations. When the writer presented preliminary data on Lord's organic oxidations in a seminar at Pennsylvania State University for Elving, he immediately saw its broad utility and later applied graphite electrode voltammograms to antioxidants (9). Fourth, when carrier-free silver in a nitrate or perchlorate medium failed to follow the writer's predictions (instead depositing several tenths of a volt more readily than expected(10,11)), other experimental variables had to be examined. The Oak Ridge studies were continued at M. I. T. on silver, by Byrne (12-14) , and on copper, by DeGeiso (15). The latter paper summarized the evidence for using interatomic distances in the substrate and in the deposit as a basis for making rough estimates of the magnitude of the underpotential - or its probable absence. Related studies. Two or three doors down the hallway from our laboratory was one in which G. E. Boyd and Q. V. Larsen were working on the chemistry of technetium, especially in the form of pertechnetate. Because per-



400

ELECTROCHEMISTRY, PAST AND PRESENT

manganate might be a useful model as well as being of interest in its own right, voltammograms were run on solutions of alkaline permanganate. Three one-electron steps were found. Controlled-potential electrolysis on the second plateau of current produced a sky-blue solution of Mn(V) with some black dioxide suspended in the stirred solution (16) . Later, studies with pertechnetate showed a black precipitate, probably the dioxide, on the electrode. Using the voltammetric data, controlled potential electrolysis under those conditions was shown to produce technetium in a form having much less molybdenum than the original preparation (17) . Hence, the voltammetric studies with platinum electrodes paid other dividends. Fortuitous experiments Perhaps the reader is wondering which of the above experiments were judged to be fortuitous - and why. The fact is that none of them would have been performed if the initial search of the literature had been successful! Before deciding to start the electrochemical research, the writer made a very thorough search of Chemical Abstracts and all treatises on electrochemistry, including chapters in books on microchemistry. No previous studies of very dilute solutions were found under titles such as electrochemistry, Nernst Equation, or electrodeposition. So our work began. About 2-3 months after we had found the phenomenon of underpotential for silver on platinum and gold, Chemical Abstracts had an abstract of an article by Haissinsky (18) dealing with electrochemistry in very dilute solutions. The article detailed the classical studies of Hevesy, Joliot-Curie and others in which the chemistry of protoactinium, RaD (Pb) , and RaE (Bi) and the corresponding daughters of thorium had been characterized in a variety of ways, including electrodeposition. Aside from the embarrassment of having missed those references, the important result was that in none of those cases had underpotential been reported! (By hindsight, one could detect marginal evidence for partial underpotential deposition only in some of the more recent data which dealt with extremely low concentrations of RaD and RaE) . However, if the writer had found those references earlier, the experiments leading to the discovery of underpotential and of quantitative anodic stripping voltammetry would not have been attempted because the usual Nernst Equation was followed. So if the experiments themselves were not fortuitous, the act of missing the references was.


26. ROGERS

Underpotential and Quantitative Anodic Stripping Voltammetry 401

Acknowledgments


The writer thanks the Department of Energy, Division of Basic Sciences, for support through Contract No.DEAS09-76ER00854 during the early stages of this manuscript. The writer also thanks his collaborators at Oak Ridge and at M. I. T. for their generous and absolutely essential contributions of ideas, criticisms, and hard work.

Literature cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

Kolthoff, I. M.; Lingane, J. J. Polaroqraphy; Interscience: New York, 1941. Laitinen, H. A.; Kolthoff, I. M. J. Phys. Chem. 1941, 45, 1061. Hickling, A. Trans. Faraday Soc. 1942, 38, 27. Lingane, J. J. Ind. Ena. Chem., Anal. Ed. 1945, 17, 332. Griess, J. C.; Rogers, L. B. J. Electrochem. Soc. 1949, 95, 129. Rogers, L. B.; Miller, H. H.; Goodrich, R. B., Stehney, A. F. Anal. Chem. 1949, 21, 777. Lord, S. S.; O'Neill, R. C.; Rogers, L. B. Anal. Chem. 1952, 24, 209. Lord, S. S.; Rogers, L. B. Anal. Chem. 1954, 26, 284. Gaylor, V. F.; Conrad, A. L.; Elving, P.J. Anal. Chem. 1953, 25, 1078. Rogers, L. B.; Krause, D. P.; Griess, J. C.; Ehrlinger, D. B. J. Electrochem. Soc. 1949, 95, 33. Rogers, L. B.; Stehney, A. F. J. Electrochem. Soc. 1949, 95, 25. Griess, J. C.; Byrne, J. T.; Rogers, L. B. J. Electrochem. Soc. 1951, 98./ 447. Byrne, J. T.,; Rogers, L. b.; Griess, J. C. J. Electrochem. Soc. 1951, 98, 451. Byrne, J. T.; Rogers, L. B. J. Electrochem. Soc. 1951, 98, 457. DeGeiso, R. C.; Rogers, L. B. J. Electrochem. Soc. 1959, 106, 433. Miller, H. H.; Rogers, L. B. Science 1949, 109, 61. Rogers, L. B. J. Am. Chem. Soc. 1949, 71, 1507. Haissinsky, M. J. Chim. Phys. 1946, 43, 21.

RECEIVED August 9, 1988


Electrochemistry, Past and Present - American Chemical Society

Recommend Documents