Studies on toluidine blue reaction with sulfite in aqueous solution and role of Cu(II) as promoter

The kinetic and mechanistic details of the reaction between toluidine blue and sulfite were studied spectrophotometrically by monitoring the depletion of toluidine blue at 633 nm. The reaction had first-order dependence on bo th the reactants, a stoichiometric ratio of 1:1 and a negative salt effect indicating the participation of TB+ and SO32- ions in the rate-limiting ste p. The reaction products were leuco base of toluidine blue and sulfate. The reaction was pH dependent and hence studied at its optimum pH 7.2. Cu(II) acted as promoter by facilitating the formation of a ternary complex with t he reactants. The protonation and stability constants of the reactants were determined and used in validating the mechanism. The activation parameters for the reactions in absence and presence of Cu(II) were determined and co mpared with other systems. (C) 1999 John Wiley & Sons. Inc.