Mechanism of the reduction of 3,7-bis(dimethylamino)phenothioniumchloride by metabisulphite ion in acidic medium

The stoichiometry, mechanism of the reduction of 3, 7-Bis(dimethylamino) phenothionium chloride known as methylene blue (here in referred to as MB) with metabisulphite ion in acidic medium have been investigated under pseudo-first order conditions at 25 ± 1oC, [H+]= 1x 10-1 mol dm-3, I = 1.0 mol dm-3 (NaClO4) and λ max = 663 nm . The stoichiometry is 1:3 and the kinetic studies indicated first order dependence in both MB and metabisulphite ion. The rate equation has been proposed as: -d/dt [MB] = k2[MB][S2O5 2- ] Where k2 = 2.06 x 10-1 mol dm-3 s-1 The rate of reaction was enhanced by increase in hydrogen ion giving the dependence of several order rate constant on [H+] as k2 = a + b[H+] and ionic strength in the reaction. Results from spectroscopic investigation and Michaelis-Menten’s plot showed no evidence of the formation of an intermediate complex. The mechanism of the reaction is therefore discussed in terms of outer-sphere mechanism.