Electrochemistry and Homogeneous Self-Exchange Kinetics of the Aqueous 12-Tungstoaluminate(5−/6−) Couple

The effect of alkali metal (M) chloride or triflate supporting electrolytes (0.1−1.0 mol L^-1) on the midpoint potential E_m of the aqueous AlW₁₂O₄₀^5-/6- couple in cyclic voltammetry, after correction (E_corr) for liquid junction potentials, can be represented in terms of ionic strength according to the extended Debye−Hückel equation. However, unrealistically short AlW₁₂O₄₀^5-/6--cation closest-approach distances are required to accommodate the specific effects of M⁺, and the infinite-dilution potential E_corr⁰ values are not quite consistent from one M⁺ to another. The pressure dependence of E_m is qualitatively consistent with expectations based on the Born−Drude−Nernst theory. The strong accelerating effects of supporting electrolytes on the standard electrode reaction rate constant k_el at pH 3 as measured by alternating current voltammetry (ACV), and on the homogeneous self-exchange rate constant k_ex at pH 3−7 as measured by ²⁷Al line broadening, depend specifically on the identity and concentration of M⁺ (Li⁺ < Na⁺ < K⁺ < Rb⁺) rather than on the ionic strength, whereas the effect of the nature of the supporting anion (Cl^- or CF₃SO₃^-) is negligible. Extrapolation of k_el and k_ex to zero [M⁺] indicates that the uncatalyzed electron transfer rate is negligibly small relative to the M⁺ catalyzed rates. The kinetic effects of M⁺ show no evidence of the saturation expected had they been due primarily to ion pairing with AlW₁₂O₄₀^5-/6-. The catalytic effect of M⁺ operates primarily through lowering the enthalpy of activation, which is partially offset by a strongly negative entropy of activation and, for the homogeneous exchange catalyzed by K⁺ or Rb⁺, becomes mildly negative; thus, the catalytic effect of M⁺ is enthalpy-driven but entropy-limited. For the electrode reaction, the volume of activation averages +4.5 ± 0.2 cm³ mol^-1 for all M⁺ and [M⁺], in contrast to the negative value predicted theoretically for the uncatalyzed reaction. These results are consistent with a reaction mechanism, previously proposed for other anion−anion electron-transfer reactions, in which anion−anion electron transfer is facilitated by partially dehydrated M⁺.