Kinetics and Mechanism of the Redox Reaction of Orange II with Sulphite Ion in Aqueous Acid

The redox reaction of orange (OR-) with sulphite ion in aqueous hydrochloric acid has been studied at an ionic strength of I = 0.50 mol dm-3 (NaCl), [H+] = 5.0 × 10-2 mol dm-3 (HCl), T = 21 ± 1oC and λ = 484 nm. The redox reaction showed that one mole of OR- reacted with two moles of SO32-. The reaction showed a first order dependence on [OR-] and [SO32-]. Changes in ionic strength of the reaction medium indicated that the rate constant increases with increase in ionic strength. The reaction was found to be independent of [H+] in the range investigated, indicating that the reaction occurs through an acid-independent pathway. Spectroscopic evidence and Michaelis - Mentens plot of 1/k1 versus 1/SO32- suggest the possible absence of an intermediate in the rate determine step. Free radical test did not yield gel formation during the course of the reaction. Added cations and anions accelerated the rate of the reaction. This effect exerted by the ions suggested that the reaction might be operating via the outersphere mechanism.