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

A 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 (Δ H # ) = 24.2 kJ M −1 ; frequency factor (A) = 1.04 × 10 4 M −3 s −1 ; and entropy of activation (Δ S # ) = −176.35 J M −1 s −1 . © John Wiley & Sons, Inc.