DOUBLE-LAYER EFFECTS IN THE KINETICS OF ELECTROREDUCTION OF ZINC(II) AT MERCURY IN DIMETHYLFORMAMIDE AND DIMETHYLSULFOXIDE SOLUTIONS

Kinetic parameters for the electroreduction of Zn(II) at mercury in dimethylformamide and dimethyl-sulfoxide solutions have been obtained in the presence of tetraalkylammonium perchlorates. Excellent agreement was found between rate constants measured by various techniques including dc and pulse polarography and cyclic voltammetry. The rate of reaction for a given supporting electrolyte was always slower in dimethylsulfoxide, a result which is attributed to the stronger solvating ability of this solvent for cations. The standard rate constant changed with the size of the cation of the electrolyte in the order ks(TPA+) > ks(TBA+) > ks (TEA+). This effect is discussed with respect to electrostatic effects in the inner layer, and compared with those seen for the reduction of other metal ions such as Cd2+ and the alkali metal ions. The corrected Tafel plots observed at potentials negative of the standard potential are curved, their slopes decreasing as the overpotential becomes more negative and then becoming constant in the most negative potential range. These observations are attributed to the electrode reaction being complex involving at least two steps. The possibility of electron transfer or ion transfer being involved in these steps is discussed. © 1990.