Kinetics of Reduction of Toluidine Blue with Sulfite—Kinetic Salt Effect in Elucidation of Mechanism

Abstract

The reaction between toluidine blue (TB+) and the sulfite ion is an ideal kinetic experiment in an undergraduate physical chemistry laboratory, to show the dependence of the reaction rate on the concentrations of reactants and ionic strength. Using excess concentrations of sulfite, the depletion of [TB+] can be monitored spectrophotometrically at 560 nm. The reaction follows pseudo-first-order kinetics. The overall reaction is second order, with first-order dependence on both sulfite and TB+ concentrations. The experiments with varying ionic strength demonstrate how kinetic salt effect can be used to predict the nature of charged species involved in the rate-limiting step. The observed negative salt effect can be interpreted as the involvement of each of TB+ and sulfite ions in the rate-limiting step. On the basis of the reaction orders and kinetic salt effect, the reaction mechanism is suggested. The theory of kinetic salt effect and some typical students' results are also described.