Kinetics and mechanism of oxidation of hydroxylamine by hexachloroiridate(IV) ion in buffer solutions

The oxidation of hydroxylammonium ion by [IrCl 6 ] 2− ion in acetic acid - acetate buffer solutions, studied by stopped flow, has the stoichiometric ratio Δ[IrCl 6 ] 2− /Δ[NH 2 OH] = 1. The oxidation involves the species [IrCl 6 ] 2− and NH 3 OH + although spectral analysis of the spent reaction mixture indicates [IrCl 5 (OH 2 )] 2− to be the main product (almost to the extent of 80%). This anomaly arises because of the aquation of the reduced product [IrCl 6 ] 3− . The rate is retarded both by H + and Cl − ions and the plots of against respective concentrations are linear. The proposed mechanism is given by reactions [i]-[v]. The values of the rate constants at 25 °C are as follows: k 1 = 147 dm 3 mol −1 s −1 and 10 2 k et K ia = 2.8 s −1 . The related activation parameters are , and, , and and , respectively. The value of K ipd is 2.91 ± 0.03 mol dm −3 and that of K ipa (= 1/K ipd ) is 0.344 ± 0.004 dm 3 mol −1 ; both values are almost independent of temperature.