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

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journal contribution
posted on 2006-11-13, 00:00 authored by Almut Czap, Nancy I. Neuman, Thomas W. Swaddle
The effect of alkali metal (M) chloride or triflate supporting electrolytes (0.1−1.0 mol L-1) on the midpoint potential Em of the aqueous AlW12O405-/6- couple in cyclic voltammetry, after correction (Ecorr) for liquid junction potentials, can be represented in terms of ionic strength according to the extended Debye−Hückel equation. However, unrealistically short AlW12O405-/6--cation closest-approach distances are required to accommodate the specific effects of M+, and the infinite-dilution potential Ecorr0 values are not quite consistent from one M+ to another. The pressure dependence of Em 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 kel at pH 3 as measured by alternating current voltammetry (ACV), and on the homogeneous self-exchange rate constant kex 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 kel and kex 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 AlW12O405-/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+.