Kinetics of the silver(I)-catalysed oxidation of H3PO2 with [ethylenebis(biguanide)]silver(III) cation in aqueous perchloric acid solution

Abstract

The silver(I)-catalysed oxidation of H₃PO₂ by [ethylenebis(biguanide)]silver(III) produces phosphite, Ag⁺ and the free ligand quantitatively. A coloured intermediate III is formed within the time of mixing which is probably composed of the silver(III) complex, Ag⁺ and hypophosphite in its active form. The first-order rate constant for the oxidation is k₀=kK₁K₂[Ag⁺][P^I]/([H⁺]+K_d), where K₁K₂ is the formation constant of III and K_d is the acid dissociation constant of H₃PO₂. Phosphorous acid does not affect the kinetics and no free radicals could be detected during the course of the reaction. The second-order rate constant KK₁K₂= 6.73 dm³ mol^–1 s^–1 at 30 °C and I= 1.0 mol dm^–3 is very much larger than the second-order rate constant reported for the oxidation of hypophosphite by silver(I). A plausible mechanism involves a two-step electron transfer from P^I to Ag^IIIvia Ag^I within III. The reaction has been utilised in developing a kinetic method for the estimation and detection of millimolar concentrations of hypophosphite; other oxoacids of phosphorus do not interfere.