Studies on Alternating Current Electrolysis: V . Double Layer Resistance and Electrode Area as Factors in Electrode Kinetics

Abstract

Impedance measurements were made on an a‐c electrolytic cell composed of platinum electrodes in poised aqueous solutions of ferrocyanide and ferricyanide containing potassium sulfate as supporting electrolyte. Empirical equations were developed for the polarization resistance and capacitance of the supporting electrolyte as functions of frequency, expressing each as the sum of frequency‐dependent and frequency‐independent terms. The frequency‐dependence of the polarization resistance proved it to be nonfaradaic in origin and hence probably a double‐layer resistance. The frequency‐independent term of the capacitance equation was shown to be altered by addition of the ferrocyanide‐ferricyanide system, contrary to the assumption usually made. This fact was employed in altering the Bockris‐Conway “v‐method” of separating the faradaic and double‐ layer admittances. This altered method proved to be more satisfactory than the “classical method.” The Bockris‐Conway theory of the double‐layer admittance was shown to be in accord with the experimental data in certain regards, but the value of the relaxation time calculated from these data was found to be of a different order of magnitude than that to be expected from the theory. An equation for the calculation of the rate constant for electron transfer at an electrode, without the necessity of knowing the electrode area, was derived. Values of rate constants calculated from this equation compared favorably with those of Randies and Somerton. Measurements made on a series of solutions of different depolarizer concentrations but constant ionic strength gave evidence that depolarizer adsorption is a disturbing influence only in solutions of concentrations less than 0.005N.