Potentiostatic and Galvanostatic Method for Fast Electrode Reactions

Abstract

Recent developments in experimental methods for investigation of fast electrode reactions are reviewed with some emphasis on author's contributions in the laboratory of Dr. Delalzay, Professor of Louisiana State University. An electrode process with a single rate-determining step that involves the same number of electrons as the complete electrode reaction can be characterized by the following equation, I0=nFksC00(0-α)C0Rα, where the notations have their usual significances. Exchange current density Io and transfer coefficient α or standard rate constant ks must be defined in order to investigate the electrode processes. These characteristic kinetic parameters are deduced from current-potential relationship under conditions for which the interpretation is as simple and rigorous as possible. The simplest condition will be achived when concentration polarization is avoided, and the kinetic parameters, in such cases, can be determined by conventional polarization experiments. However, the conventional polarization methods are limited to the applications for the studies of very fast elec rode reactions, If exchange current is large enough, mass transfer is determinative, and the Nernst equation is obeyed for all practical purposes; the overvoltage cannot be measured and the determination of the kinetic parameters will be impossible. In such fast electrode reactions, it will be inadequate to use the conventional methods, and the so-called relaxation methods should be applied for determination of the krnetic parameters. In relaxation methods, the working electrode potential is disturbed from its equilibrium value for a very short time and kinetic characteristics of the electrode are measured. In practice, concentration polarization cannot be completely avoided but can be minimized by fast observation of the electrode responce, and partial or total control of the electrode process by mass transfer will be avoided. Perturvations represented by a step function or a periodic function of time have been used, and the methods can be classified into three groups of potentiastatie, galvanostatic, and A.G. methods. Potentiostatic method and galvanostatic method will be discussed here, and details of the instrumentation for these methods are also described. Potentiostatic method.-In this method, the electrode potential which is initially at its equilibrium value is abruptly charged, and the current through the cell is observed. Exchange current is obtained from a linear plot of current against the square root of time, and the transfer coefficient is determined from a log-log plot of exchange current against the varying concentration of one reactant. Galeanostatic method.-In this method, the current through the cell is abruptly changed and kept constant even if internal resistance of the electrolytic cell changes quickly or gradually, and the potential of working electrode is recorded on oscilloscope or high pen speed recorder. The exchange current is calculated from the linear plot of overvoltage against the square root of time, and transfer coefficient α is deduced from a log-log plot of exchange current against the concentration of one reactant. These methods for fast electrode reactions were also applied to investigations of the so-called kinetic processes when chemical reaction preceding charge transfer is too fast to be studied by conventional polarization methods. The kinetics of dissociation of monochloroacetic acid in 50-50 water-ethanol mixture (in volume per cent) was studied by potentiostatic method. For faster electrode reaction such as the discharge of mercurous ion on mercui y electrode, the errors in the extrapolation to an initial current or voltage value become increasingly serious.