Kinetics and Mechanism of the Oxidation of Orange II by Heptaoxodichromate(VI) Ion in Aqueous Acid

The kinetics of the oxidation of orange II (here and thereafter referred to as OR-) by heptaoxodichromate(VI) ion has been studied spectrophotometrically at constant ionic strength, I = 0.50 C2mol dm-3 (NaCl), H+ = 5.0  10-2mol dm-3 (HCl) and T = 24 1C. The redox reaction displayed a stoichiometry of 2:1 andrate equation for the reaction is:-d[OR-]/dt= (a + b[H+])[OR-][Cr2O72-]. The rate of the reaction increases with increase in acid concentration. Varying the ionic strength of the medium from 0.2 – 0.9 C2mol dm–3 (NaCl) showed that the rate constant increases with increase in ionic strength, suggesting a positive BrØnsted - Debye salt effect. The reaction shows a first order dependence on [oxidant] and [reductant]. Added ions (Ca2+, Mg2+, CH3COO- and SO42-) on the reaction rate increased the rates of the reactions. Michaelis–Menten’s plot of 1/k1 versus 1/ Cr2O72-and spectrophotometric test suggest absence of an intermediate in the rate determining step. Polymerisation was not induced by the addition of acrylonitrile to reaction mixture followedby excess methanol, as evidenced by the formation of a gelatinous precipitate. The results obtained suggest the absence of the formation of intermediate complex thereby supporting the outersphere mechanism.