A kinetic study of the reduction of toluidine blue with thiourea in acidic solution

Abstract

AbstractA detailed kinetic study of the reaction of toluidine blue (tolonium chloride) (TB+ Cl−) with thiourea (TU) in aqueous hydrochloric acid solution is reported. The reaction was first order with respect to toluidine blue and the reductant and second order with respect to [H+]. Thiourea had a 2:1 stoichiometric ratio with TB+. Toluidine blue was reduced to a colorless base in two one‐electron reduction steps and TU was oxidized to thioformamidinium ion, which dimerized rapidly to give stable dithioformamidinium ion. The energy parameters obtained for TB+‐TU reaction were mean energy of activation (Ea′) = 26.7 ± 2.4 kJ M−1; enthalpy of activation (ΔH#) = 24.2 kJ M−1; frequency factor (A) = 1.04 × 104 M−3 s−1; and entropy of activation (ΔS#) = −176.35 J M−1 s−1. © John Wiley & Sons, Inc.