Kinetics of the base decomposition of dodecatungstophosphate(3–) in weakly alkaline solutions

Abstract

The kinetics of the decomposition of [PW₁₂O₄₀]^3–(1.5 × 10^–5 mol dm^–3) to [WO₄]^2– and [PO₄]_3– has been studied in the range pH 7–9 at an ionic strength of 0.1 mol dm^–3. The reaction proceeds via the intermediates [PW₁₁O₃₉]^7– and [PW₉O₃₄]^9–. Only the final stage, the hydrolysis of [PW₉O₃₄]^9–, was studied in detail. The [PW₉O₃₄]^9– ion exists in rapid equilibrium with [HPW₉O₃₄]^8– and [H₂PW₉O₃₄]^7– with the unprotonated and monoprotonated ions decomposing via first-order kinetics. The pK_a values for [H₂PW₉O₃₄]^7– and [HPW₉O₃₄]^8– were determined as 7.3 and 9.2 respectively, and the rate constants for decomposition of [HPW₉O₃₄]^8– and [PW₉O₃₄]^9– as 8.0 × 10^–4 and 6.3 × 10^–3 s^–1 respectively at 25.6 °C.