Pressure Dependence of the Electrode Potentials of Some Iron(III/II), Cobalt(III/II), and Ni(III/II) Couples in Aqueous Solution: Estimation of the Reaction Volumes for M(III)/M(II) Couples

Pressure, Electrode Potential, Reaction Volumes Pressure dependence of the electrode potentials for various M3+/2+ couples (M = Fe, Co, and Ni with o-phenanthroline, bipyridine, terpyridine, 1,4,7-triazacyclononane, and 1,4,7-trithiacyclononane) was measured by cyclic voltammetry in aqueous solution vs. a Ag/AgCl reference electrode. The reaction volume of the Ag/AgCl reference electrode was estimated from the change in the partial molar volumes of Feaq3+/2+ and Co(terpyridine)23+/2+ couples, and the volumes for the formation of M2+ from M3+ were determined. The electrostrictive component for the change in the oxidation state for the M3+/2+ couple was calculated on the basis of the Drude-Nernst equation and by the mean spherical approximation (MSA). The isolated volumes for the formation of M 2+ from M 3+ were compared with the values predicted by theory.