Kinetics and mechanism of oxidations by peroxodiphosphate ions. Part 3. Oxidation of hypophosphite in aquous perchloric acid by the hydrolytic product peroxomonophosphate in a consecutive reaction

Abstract

Oxidation of hypophosphite by peroxodiphosphate in perchloric acid solution is a consecutive reaction, (i) and (ii), H₄P₂O₈+ H₂O [graphic omitted] H₃PO₅+ H₃PO₄(i), H₃PO₅+ H₃PO₂ [graphic omitted] H₃PO₄+ H₃PO₃(ii) and a plot of [H₃PO₅] against time for the reaction exhibits a maximum. In a separate study, the rate law for reaction (ii) has been found to be (iii), where K₁ and K₂ are the acid-ionization constants of H₃PO₂ and H₃PO₅–d[H₃PO₅]/dt=k₂[H₃PO₅]_T[H₃PO₂]_T[H⁺]²/([H⁺]+K₁)([H⁺]+K₂)(iii) respectively; k₂ was (1.2 ± 0.08)× 10^–2 dm³ mol^–1 s^–1 at [H⁺]= 0.50 mol dm^–3, I= 1.0 mol dm^–3, and 45 °C. An estimated value of K₂ is 0.15 mol dm^–3 at 35 °C and I= 1.0 mol dm^–3. Maximum concentrations of H₃PO₅ and the times at which they are built up have been calculated using the values of k_h and k₂, and these are in good agreement with the observed values.