Voltammetric reduction of cations in mixed solvents

Abstract

By using a mercury-pool indicator and a molybdenum-wire reference electrode, the reduction of various ions, including Cd²⁺, Tl⁺, Eu³⁺, CrO^2–₄, was studied on fast voltage-scanning instruments. The shifts in the reduction potentials were studied for different mixtures of water with methanol, propanol, acetone, methyl cyanide, dioxan and formamide. For all the ions under test except CrO^2–₄, in every solvent except water-formamide, the shift is towards negative potentials. In water-formamide the shift is towards more positive potentials. The relevant factors determining the shift in potential, such as solvation, ion-pair formation, increased resistance of the solution, adsorption, structure of the solvent and composition of the supporting electrolyte, were considered. The contribution of factors other than solvation was found to be minimal. For the ions considered it was found that the changes in the activity coefficients of the metal ions for various aqueous mixtures of organic solvents play an important part in shifting the reduction potential. The theoretical shifts were calculated and compared with the experimental observations. The trend in both cases was found to be in the same direction.