Kinetics of the Hg(II)/Hg electrode reaction in DMSO solutions of chloride ions

The electrode reaction Hg(II)/Hg in complex chloride solutions with dimethyl sulfoxide as solvent has been investigated at the equilibrium potential by the faradaic impedance method and a cyclic current-step method. The ionic strength was 1 M with ammonium perchlorate as supporting electrolyte, and the temperature was 25°C. Double-layer data have been determined by electrocapillary measurements. From the results of the kinetic measurements at ligand numbers ≤1.1 or ≥2.3 it is concluded that the overall charge transfer proceeds step-wise. The solvated Hg²⁺ and Hg₂²⁺ as well as the complexes HgCl_j^2?j and the dinuclear Hg₂Cl³⁺ contribute to the exchange current density. The rate constant of the step HgCl_j^2?j/ Hg(I) is found to increase with the number of Cl^? coordinated. This increase can be correlated to a decrease in solvation and a lengthening of the Hg?Cl distance. For 1.1 << 2.3, impedance measurements indicate a rate-controlling adsorption step. It is suggested that the uncharged HgCl₂ then forms an adsorbed network on the mercury surface.