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(corr)(0) 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 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 cm(3) 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+.