On the role of outer solvation shell in simple electron transfer reactions. Electrode processes of the Co(en)33+/2+ couple in selected mixed solvents

The electrode reaction of the Co 3 3+/2+ couple on mercury electrodes in water-acetonitrile (AN), water-DMSO and water-hexamethylphosphortriamide (HMPT) mixed solvents has been investigated. The differences between Gibbs energies of trasfer of the Co(en) 3 3+/2+ and Co(en) 3 2+ ions from water to mixtures studied were calculated from the formal potentials expressed in the ferrocinium ion/ferrocene (Fic + /Foc) scale. The data obtained point to selective solvation by more basic organic solvent in the second sphere. The determined dependence of electrochemical rate constant on mixed solvent composition is different from that observed in the case of simple solvated ions involving electron transfer, mainly due to be reactant adsorption from aqueous solutions. The results obtained were useful in extension of the model of electrochemical reactions in mixed solvents, proposed earlier for simple inorganic ions.