Kinetics and Mechanisms of Toluidine Blue Reduction by Dithionite Ion in Aqueous Acidic Medium

S. A. Hamza, J. F. Iyun, S. O.

Abstract

Kinetics of reduction of toluidine blue (hereafter referred to as TB+) by sodium dithionite in aqueous hydrochloric acid medium has been studied spectroscopically at λ = 600 nm. The order of the reaction with respect to TB+ and dithionite ion was found to be one and 1.39 respectively. The rate of the reaction increased with increase in [H+]. Variation of the ionic strength and dielectric constant of the medium altered the rate of the reaction, addition anions (X = SO4 2-, NO3 -) to the reaction mixture decreased the rate of the reaction. Spectroscopic and kinetic investigation showed no sign of intermediate complex formation; free radical polymerisation test showed the presence of free radicals. A plausible mechanism which accommodates all the experimental data was proposed. The rate law for the reaction conforms to the equation: Rate = k4K2[TB+] [S2O4 2-][H+] + k5 k3K1 ½[TB+] [S2O4 2-]½[H+]