The role of the supporting electrolyte cation in the kinetics of outer-sphere electrochemical redox processes involving metal complexes

Abstract

The electroreduction rates of Cr(OH2)63+ and Cr(NH3)63+ at the mercury–aqueous interface were monitored as a function of electrode potential in electrolytes containing univalent, divalent, or trivalent cations with systematically varying atomic number in order to scrutinize the role of the supporting electrolyte cation in the kinetics of these simple one-electron outer-sphere electrode reactions. The comparison between the corresponding rate responses for Cr(OH2)63+ and Cr(NH3)63+ reduction brought about by changing the supporting electrolyte cation is expected to provide information on the influence of the cation upon the potential profile in the double layer; despite the close structural and mechanistic similarity of the reactants, the transition state for the latter reaction appears to lie significantly closer to the electrode than that for the former process. The virtues of comparing rate responses at a constant electrode charge density rather than at a constant electrode potential are pointed out. It was found that the variations in the effective average potential at constant charge [Formula: see text]ding changes in the electrode potential [Formula: see text] derived from thermodynamic double-layer measurements. These results, along with the observed variations in the transfer coefficients for the two reactions, are semiquantitatively consistent with a model involving changes in the position of the oHp as the supporting electrolyte cation is altered, but are inconsistent with a model attributing the rate variations to cation specific adsorption.