Electrostatic effect of specifically adsorbed electro-inactive ions upon electrode processes: I. Tetrathionate electroreduction in the presence of Br −, I −, SCN −, N 3− and Tl +

Abstract

Summary The reduction rate k f of S 4 O 6 2− in the presence of halides and pseudohalides at sufficiently negative potentials with respect to the point of zero charge satisfies the well-known Butler-Volmer equation corrected for diffuse-layer effects according to Frumkin. On the other hand at less cathodic potentials, where halide and pseudohalide adsorption becomes appreciable, the rate constant k f at constant applied potential E , once corrected for diffuse-layer effects according to Frumkin, still depends on the charge density q i due to the specifically adsorbed halide and pseudohalide ions. Thus in the presence of Br − , I − , SCN − , and N 3 − , the logarithm of k f, d =0 (where k f, d =0 denotes the rate constant at constant E corrected for diffuse-layer effects) decreases linearly with | q i |, at least for | q i |>∼10 μC cm −2 . Analogously, in the presence of the specifically adsorbed Tl + ion, log k f, d =0 increases linearly with the positive charge density q i due to this cation. The slopes of the various log k f, d =0 vs . q i plots are in satisfactory agreement with a theoretical treatment proposed by the authors 5,7 , which accounts for the electrostatic interactions between the activated complex for the electrode reaction and the neighbouring specifically adsorbed electroinactive ions within the compact layer.