Oxidation of thiosulphate by hexacyanoferrate(III) in aqueous perchloric acid medium - A kinetic and mechanistic study

The kinetics of oxidation of thiosulphate by hexacyanoferrate(III) in an aqueous perchloric acid medium has been studied spectrophotometrically by stopped flow method at 25 degrees C. The stoichiometry is 1:1, i.e., one mole of thiosulphate consumes one mole of hexacyanoferrate(III). The reaction products are identified as Fe(CN)(6)(2-) and S4O62-. The reaction is first order with respect to hexacyanoferrate(III) and thiosulphate concentrations. Increase ill perchloric acid concentration increases the rate of reaction (order=0.40). Added products do not have any significant effect on the rate of reaction. The effect of ionic strength and dielectric constants oil the reaction rate has also been studied. The active species of oxidant is indicated to be HFe(CN)(6)(3-). A suitable mechanism is proposed and the reaction constants of the different steps involved have been evaluated. Activation parameters have also been calculated with respect to the slow step of the mechanism.