Solution chemistry of polyanions

An approach using ion-transfer voltammetry

Abstract

Heteropolyanions are widely used as acid and redox catalysts for various synthetic reactions. Also, their formation reactions have been the basis of the widely utilized methods for the determination of such oxoanions as phosphate and silicate ions. In such analytical processes, either water-immiscible or water-miscible organic solvents have been used very effectively as extraction solvents and auxiliary solvents for stabilizing heteropoly complexes. Although many heteropolyanions possess hydrophobic characterisctics, there has been no available measure for evaluating the affinity of heteropolyanions to organic solvents. Over the past several years, we employed ion transfer voltammetry to study the transfer of heteropoly- and isopolyanions at the oil/water interfaces, viz., nitrobenzene/water and 1, 2-dichloroethane/water interfaces. The standard ion transfer potentials(Δ^W_Oφ^_??_)of the polyanions tested were found to depend linearly on the quantity |z|/n^2/3 (z, ionic valence, n, number of oxygen atoms), which can be regarded as being proportional to the surface charge density of a spherical polyanion. On the basis of this result, we proposed the value |z|/n^2/3 to be used for a hydrophobicity scale of polyanions. This scale may provide a useful criterion for evaluating the hydrophobic (or hydrophilic) properties of polyanions, including the stability in solutions and extractability into organic solvents. The linear dependence of Δ^W_Oφ^_??_ on the surface charge density provides significant information regarding ion solvation energy as well. This dependence means that the Gibbs transfer energy depends linearly on 1/γ² (γ, ionic radius), and it cannot be accounted for by the Born-type electrostatic solvation energy, which should depend on 1/γ. Such a non-Bornian solvation energy may be considered as the short-range interactions of an ion with solvent molecules (i.e., donor-acceptor effects or hydrogen bonds), which seem to play the most important role in the ion transfer energy. In addition to the contribution to solution chemistry, a few contributions have been made to coordination chemistry and electroanalytical chemistry, i.e., the characterization of aqueous polyanion solutions and the development of a novel voltammetric phosphate sensor.