Electrochemical Studies at Elevated Pressures: Reaction Volumes of Some Fe (III/II), Co (III/II), and Ni (III/II) Couples in Aqueous Solution and in Acetonitrile

Abstract

Pressure dependence of the electrode potentials for various M^3+/2+ couples (M=Fe, Co, and Ni) was measured in aqueous solution and in acetonitrile with Ag/AgCl and Ag⁺/Ag reference electrodes. The reaction volumes of reference electrodes were estimated from the change in the partial molar volumes of Fe_ap^3+/2+ and Co (terpyridine) ₂^3+/2+ couples, and the reaction volumes corresponding to the formation of M (II) from M (III), ΔV°_M, were isolated. The applicability of the MSA model to the electrostrictive contribution to the reaction volumes was examined. It was shown that the MSA model explains the experimantally obtained reaction volumes well for the redox couples in solution, although the predicted interaction distances between the solute ion and solvent molecules are rather small.