No abstract

The first and second redox potentials ( and ) of Keggin-type heteropolytungstate anions (α-[PW12O40]3− and α-[SiW12O40]4−) have been determined in various solvents by means of cyclic voltammetry. The difference () between and , being theoretically associated with the solvation energies of the polyanions, does not correlate with the permittivity of the solvent, in conflict with an expectation based on the Born-type electrostatic solvation model. In contrast, has been found to show some definite correlations with such empirical solvent parameters as acceptor numbers. This shows that the short-range interactions (i.e., donor–acceptor effects or hydrogen bonds) between the heteropolyanion and its primary solvents play the most significant role in the solvation of the heteropolyanions. We have thus proposed a new analytical equation for evaluating the short-range ion–solvent interaction energy.

A new liquid/liquid-type reference electrode for ion-transfer voltammetry with the nitrobenzene (NB)/water (W) interface has been developed. This reference electrode is expressed as Pt/1.8 mM α- [SiMo12O40] 4-+ 0.2 mM α-02 M tetrahexylammonium tetraphenylborate (NB). The Galvani potential difference between the reference NB/W interface is determined virtually by the distribution equilibrium of the heteropolyanion (α- [SiMo12O40] 4-), whereas the Pt electrode potential is determined by the redox equilibrium of the heteropolyanion couple. With this reference electrode, a potential window of about 0.4 V is available in a fairly negative potential region, where the transfer of hydrophilic inorganic anions (Br