Half-wave potential as affected by supporting electrolyte nature: Interplay of adsorption and ionic association for electroreduction of V(V)-mixed addenda Keggin tungstophosphate

V(V)-mixed addenda Keggin tungstophosphate demonstrates the pronounced dependence of polarographic half-wave potential E1/2 at positively charged mercury surface on the nature of supporting alkali metal cation. This effect (typical for polyoxometalates) complicates the straightforward application of E1/2 values to analysis of various thermodynamic trends and correlations. Cation-induced E1/2 shifts are explained under assumption of Keggin anions reduction from compact adlayers containing the cations. Strong adsorption of the reactant is confirmed by a decrease of interfacial surface tension and by pronounced inhibitory effect on the outer-sphere electron transfer reactions. Stabilization of adlayers by co-adsorbing supporting cations increases in a series Li+ < Na+ < K+, as follows from surface tension measurements and inhibition of peroxodisulfate reduction. The effect of cations on electrochemical behavior of non-mixed addenda Keggin tungstophosphate is discussed for comparison.