Kinetics and mechanism of osmium(VIII)-catalyzed oxidation of hypophosphite by hexacyanoferrate(III) in aqueous alkali

Abstract

The kinetics of osmium(VIII)-catalyzed oxidation of hypophosphite with hexacyanoferrate(III) in alkaline medium has been studied. The rate is independent of the concentration of the oxidant. The order with respect to hydroxide ion is variable. Rate law (1) conforms with the experimental observations. $$ - \frac{{{\text{d[Fe(Cn)}}_{\text{6}}^{{\text{3 - }}} {\text{]}}}}{{{\text{d}}t}} = \frac{{2kK_1 K_2 [{\text{Os}}^{{\text{VIII}}} ][{\text{H}}_{\text{2}} {\text{PO}}_{\text{2}}^{\text{ - }} ][{\text{OH}}^{\text{ - }} ]}}{{1 + K_1 [{\text{OH}}^{\text{ - }} ]}}$$ The equilibrium constant 'K1' for step (2) $$[{\text{OsO}}_{\text{3}} ({\text{OH}})_3 ]^ - + {\text{OH}}^ - \mathop \rightleftharpoons \limits^{K_1 } [{\text{OsO}}_{\text{4}} ({\text{OH}})_2 ]^{2 - } + {\text{H}}_{\text{2}} {\text{O}}$$ has been evaluated kinetically to be (21 ± 5.0), (23 ± 5.0), (26 ± 6) and (32 ± 6) at 25, 30, 32 and 35 °C and I = 1.0 mol dm−3 respectively. The energy and entropy of activation were calculated to be (42 ± 2.0) kJ mol−1 and (82 ± 6.0) J K−1 mol−1 respectively. A plausible reaction mechanism has been suggested.