Electrochemical characteristics of the iron(III)/iron(II) system in dimethylsulphoxide solutions

A detailed study of the electrochemical characteristics of the iron(III)/iron(II) redox couple has been made in non-complexing perchlorate and complexing chloride media in dimethylsulphoxide (DMSO) solutions and their aqueous analogues. The formal potentials of the Fe(III)/(II) couple in both media in DMSO are approximately 0.5 V less positive than their aqueous counterparts. This is a reflection of the very much stronger solvation of Fe(III) cf. Fe(II) by DMSO. The present results also indicate, contrary to previous studies, that the chloro-complexes of Fe(II) and Fe(III) are of similar strength. Diffusion coefficients for Fe(II) and Fe(III) in DMSO solutions were found to be an order of magnitude lower than their aqueous analogues as a result of the higher viscosity of DMSO. Heterogeneous (electron transfer) rate constants for the Fe(III)/(II) system on platinum were also found to be an order of magnitude less in DMSO solutions the in the corresponding aqueous solutions. This was attributed to the specific adsorption of DMSO on the platinum electrode. Cyclic voltammetry indicated that the Fe(III)/(II) electrode reaction in both ClO−4 and Cl− media in DMSO involves a quasi-reversible electron transfer without any coupled chemical reactions