KINETICS OF THE SILVER(I)-CATALYZED OXIDATION OF H3PO2 WITH [ETHYLENEBIS(BIGUANIDE)]SILVER(III) CATION IN AQUEOUS PERCHLORIC-ACID SOLUTION

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(l)]/([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-degrees-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(l) to Ag(III) via 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.