Electrostatic effect of specifically adsorbed electroinactive ions upon electrode processes: Part VI. Dimethylglyoxime electroreduction from weakly acidic aqueous solutions in the presence of I− and SCN−

An investigation of the _d-effect in the absence of ionic specific adsorption shows that the overall charge of the reactants in dimethylglyoxime (DMG) electroreduction on mercury from aqueous solutions of pH?2 equals+1, in agreement with a protonation reaction either preceding (DMG + H⁺DMGH^++e → products) or else simultaneous with (DMG + H⁺ + e → products) the charge transfer. In the presence of the specifically adsorbed supporting anions I^? and SCN^? the logarithm Φ of the rate constant for DMG electroreduction corrected for diffuse-layer effects decreases linearly with an increase in the absolute value |q_i| of the charge density at the inner Helmholtz plane due to the adsorbed anions. This behaviour is explained semiquantitatively by assuming that protonation takes place simultaneously with charge transfer, with the DMG molecule in the adsorbed state and the H₃O⁺ ion in the non-adsorbed state. In this way the H₃O⁺ ion, due to the screening effect of the surrounding diffuse-layer ions, escapes the accelerating attractive effect exerted by the adsorbed I^? or SCN^? ions; on the other hand these latter anions exert a retarding repulsive effect upon the electron transfer from the metal to the adsorbed DMG molecule.