Electrode kinetics of the ferrocyanide-ferricyanide couple

At first sight, the redox reactions of the hexacyano-ferrate species in aqueous solutions on noble metal electrodes are fast, simple, outer-sphere electron transfer with no intermediates. Accordingly, the cyclic voltammograms, CVs, of this system are of the form of “quasi-reversible”, diffusion-affected charge transfer. However, it is known for decades that redox potential and electron exchange rates depend on the cation of the base electrolyte and on its concentration. This implies some complexity in this redox process. To explore the kinetics and reaction mechanism, CV and dynamic electrochemical impedance spectroscopy measurements have been performed in ferrocyanide– or ferricyanide–containing base electrolytes of varied concentrations. The CVs were analysed using Pajkossy's recently elaborated procedure for decoupling diffusion and charge transfer. This procedure enables the determination of charge transfer rate coefficients for both oxidation and reduction processes separately. The rate of the reduction process was found to be proportional to the concentration, whereas the oxidation rate is independent of it. Based on this finding, the following mechanism is suggested: the ferrocyanide oxidation proceeds directly, whereas the formation of an associate species with a Li + cation precedes the ferricyanide reduction.