Electroreduction of peroxodisulfate on mercury in mixed water-carbohydrate media: The interplay of solvent effects and concentration-dependent structure of reaction layer

DC polarography is applied to study the effect of sucrose and glucose concentration in aqueous solutions on the rate of peroxodisulfate electroreduction. Non-monotonous dependence of reduction rate (current) on viscosity is found, which apparently contradicts the dependences expected for both diabatic and adiabatic reactions. The procedures of experimental data treatment are proposed, aimed to separation of theoretically predicted solvent effects (both static and dynamic) from various effects of solvent-dependent reaction layer structure. The latter are unavoidable because of concentration-dependent adsorption of carbohydrates and the dependence of electrostatic behavior of ions on solvent composition. Moreover, contribution of these unavoidable side effects is rather pronounced, and can affect the conclusions concerning solvent effect even qualitatively. Data treatment consists in successive consideration of ion pairing in solution bulk, partial surface blockage with adsorbed co-solvent molecules, and electrode-reactant repulsion, with step-by-step correction of initial current-viscosity dependences. In the course of this treatment, two limiting cases of adsorption layer geometry were always considered, and various assumptions concerning the reactant localization were applied, to avoid any uncertainty and to estimate the accuracy of corrections. We conclude solidly that non-monotonous current dependence on carbohydrate concentration does not result from any side effects and should be attributed to the interplay of dynamic and static solvent effects, being of different sign. Finally, we formulate the problems for further theoretical studies related to interpretation and prognosing of solvent effects for real systems.