Kinetics and mechanism of the redox reaction of orange II with thiosulphate ion in aqueous acid

Abstract

The kinetics of the redox reaction of orange II (OR-) with thiosulphate ion has been studied spectrophotometrically at constant ionic strength, I = 0.50 mol dm-3 (NaCl), [H+] = 5.0 X 10-2 mol dm^-3 (HCl) and T = 26 ± 1°C. The redox reaction displayed a stoichiometry of 1:4 and rate equation for the reaction is
-d[OR-]/dt = (a + b[H⁺])[OR-][S₂O₃^2-]
The rate of the reaction increases with increase in acid concentration and in the ionic strength of reaction medium. The reaction shows a first order dependence on [oxidant] and [reductant]. Added cations and anions inhibited the rate of the reaction. Michaelis – Menten’s plot of 1/k₁ versus 1/ S₂O₃^2- and spectrophotometric test suggest absence of an intermediate in the rate determining step. Free radical test did not yield gel formation. Based on the results obtained, this reaction is probably operating through the outersphere mechanism.