Kinetics of the dissociation of decavanadate(6–) in neutral and weakly basic solutions

Abstract

The kinetics of decomposition of [V₁₀O₂₈]^6–(2 × 10^–3– 9 × 10^–5 mol dm^–3) have been studied spectrophotometrically and pH-metrically within the range pH 6–12.5 at 25 °C, /= 0.1 mol dm^–3, and 0.1 mol dm^–3 Na⁺ cationic medium. A treatment of the pH dependence of the observed rate constants demonstrates the presence of fast acid-base pre-equilibria involving the reactive species [V₁₀O₂₈]^6–. Protonation leads to the well known and stable species [HV₁₀O₂₈]^5– and decreases the overall decomposition rate. Deprotonation gives rise to an unknown and more reactive [HV₁₀O₂₉]^7– species, increasing the rate. The equilibrium constant for formation of the [HV₁₀O₂₉]^7– species from hydrated [V₁₀O₂₈]^6– and hydroxide is 1.06 × 10⁵ dm³ mol^–1 The rate of decom-position of decavanadate is given by the contributions from two first-order paths, k₁[V₁₀O₂₈^6–]+k₁′[HV₁₀O₂₉^7–], where k₁= 1.6 × 10^–5 and k₁′= 1.02 × 10^–4s^–1.