Capillary electrophoretic study on the formation and transformation of isopolyoxotungstates in aqueous and aqueous-CH3CN media

Abstract The formation and transformation processes of isopolyoxotungstates have been studied in aqueous and aqueous-CH 3 CN solutions of 5.0 mM W(VI), using capillary electrophoresis (CE). In freshly prepared aqueous media of pH 2.0–5.0, five migration peaks were identified as [W 10 O 32 ] 4− , ψ-metatungstate, β-[H 2 W 12 O 40 ] 6− , α-[H 2 W 12 O 40 ] 6− and [W 7 O 24 ] 6− . In the pH region of 2.0–4.0, the time-dependence of each peak area corresponds to the spontaneous direction of transformation: [W 10 O 32 ] 4− →ψ-metatungstate→β-[H 2 W 12 O 40 ] 6− →α-[H 2 W 12 O 40 ] 6− . At higher pH values, [W 7 O 24 ] 6− also showed the same sequence of transformation: [W 7 O 24 ] 6− →ψ-metatungstate→β-[H 2 W 12 O 40 ] 6− →α-[H 2 W 12 O 40 ] 6− . In 40% (v/v) CH 3 CN–water media, [W 10 O 32 ] 4− and [W 6 O 19 ] 2− were kinetically stable in the buffer pH range of 2.0–3.5. By heating the aqueous-CH 3 CN solutions at buffer pH values of 4.0–5.0, a mixture of α-[H 4 W 12 O 40 ] 4− and [W 7 O 24 ] 6− was formed. On the other hand, ψ-metatungstate, β-[H 2 W 12 O 40 ] 6− and α-[H 2 W 12 O 40 ] 6− did not exist at any appreciable concentrations. Their formation conditions were established as functions of the buffer pH, the reaction time, the reaction temperature and the CH 3 CN concentration.