Kinetics and mechanism of redox reactions in aqueous solution. Part 2. Oxidation of phosphorous acid by vanadium(V) ion

Abstract

The redox reaction between V^V and H₃PO₃ in the presence of Ag^I has a zero-order dependence on [V^V]. There is a linear relation between k₀^–1 and [Ag^I]^–1 and the plot has an intercept on the rate axis. In the absence of Ag^I the reaction exhibits a first-order dependence on [V^V]. Plots of k₁ against [H₃PO₃] and k₁^–1 against [H₃PO₃]^–1 are linear and pass through the origin. These plots, however, show some deviation from linearity at higher [H₃PO₃]. The correlations between the rate and various acidity scales are discussed. A change in the reaction medium from water to D₂O did not affect the rate; k_D/k_H < 1. Based on these results and other evidence reported in the literature, it is concluded that the reaction does not involve participation of the active: P(OH)₃ tautomer in the initial stages of the reaction. A mechanism based on the formation of V^V–H₃PO₃ complex(es) is proposed but the question whether the complex is an intermediate remains unresolved.