Medium effect on the electrochemical behaviour of the CdII/Cd (Hg) system in propane-1,2-diol–water mixtures

Abstract

The electrochemical behaviour of the Cd^II/Cd(Hg) system in propane-1,2-diol (PD)–water mixtures containing 5.0 × 10^–2 mol dm^–3 LiClO₄ has been studied by polarography and cyclic voltammetry. The reversible half-wave potentials, the diffusion coefficients and the Walden products for Cd^II have been determined polarographically. From the half-wave potentials vs. the ferrocene electrode scale, the standard free energies of transfer of 1 mol of Cd^II ions from water to PD–water mixtures, ΔG^°_t, have been obtained. These values are always negative, indicating a greater stability of Cd^II in PD-water mixtures than in pure water. The splitting of the ΔG^°_t values into electrostatic and chemical contributions shows that the mixtures are more basic than water, increasing the Cd^II solvation when the PD mole fraction rises. The analysis of the variation of the Walden product with solvent composition indicates an enhancement of the solvent structure in the water-rich region. The diffusion coefficient for Cd in mercury, the transfer coefficients for Cd^II electroreduction and the apparent standard rate constants of the Cd^II/Cd(Hg) system have been determined by cyclic voltammetry. The change in the kinetics with solvent composition is discussed in terms of existing models. The results obtained are compared with those previously reported in ethylene glycol–water mixtures.