Contemporary understanding of the peroxodisulfate reduction at a mercury electrode

Specific aspects of the kinetics of anion electroreduction at high overvoltages are addressed by various traditional procedures to treat experimental data. The expansion of the Frumkin correction concept is proposed in terms of the reaction volume which increases with increasing negative electrode charge for anionic reactants. A molecular level approach based on quantum chemical calculations of the work terms and the electrode–reactant electronic coupling is employed for calculations of the reaction volume for the case of peroxodisulfate S2O82− and the ion pair Na+·S2O82−, the species which are to be considered as possible reactants in Na2S2O8 solutions with sodium salts as supporting electrolytes. Estimates of the partial rate constants for the anion and ion pair are reported as well. New experimental data are presented for solutions of equal ionic strength and equal degree of association but with different total reactant concentration. These data confirm the possibility of local ion pair formation at low negative charge. In parallel the previously ignored problem of correction for mass transport limitations is discussed for a system with two simultaneously discharging species of various charges. It follows from this reconsideration that previous data cannot be interpreted for sure as the absence of an ion pair contribution to the total current. The problem of separation of contributions from corresponding parallel steps to the total current is discussed, and the fast formation of an ion pair preceding the electron transfer was found to be rather probable.