Electrode kinetics and double-layer structure II. The potential-dependence of the kinetic parameters of the Zn2+/Zn(Hg) electrode reaction

Abstract

The impedance of the Zn2+/Zn(Hg) electrode in 1 M KI, KBr, KCl and NaClO4 solutions has been measured as a function of potential, using an externally polarised dropping mercury electrode. The potential-dependence of the faradaic impedance has been carefully analysed. It is shown that, in general, both the apparent rate constant and the apparent transfer coefficient are potential-dependent. In the cathodic potential region (−1.05 to −1.35 V vs. SCE) the results are in accordance with a constant value, β=0.19±0.02 for the “true” cathodic transfer coefficient and a potential-dependent rate constant which increases if the amount of specifically adsorbed anions increases. In the anodic potential region (−0.90 to −0.95 V vs. SCE) a significantly different transfer coefficient is found, β=0.60±0.03, which indicates that the mechanism of the electrode reaction depends on the d.c. potential. It is suggested that the interpretation of kinetic parameters and the testing of simple theories with such systems should be approached with caution.