Kinetics of reduction of Maxilon blue-SG by sulfide ion: Kinetic salt effect in elucidation of reaction mechanism

Abstract

The reaction between Maxilon blue $(MB^+)$ and the sulfide ion is an ideal kinetic experiment in order to show the dependence of the reaction rate on the concentrations of reactants, ionic strength and temperature. By using excess concentrations of sulfide, the reaction or the disappearance of $(MB^+)$ was monitored spectrophotometrically and kinetically at 654 nm by measuring the decrease in absorbance of $(MB^+)$ . The indicator reaction follows pseudo-first-order kinetics. Specifically, the reduction of $(MB^+)$ with $S^{2-}$ ion in the presence of Se(IV) in an phosphate buffer medium, pH=6.5 was studied. The rate law for this system was found to be $rate_i=k_{cat}[MB^+][S^{2-}]^3[Se(IV)] + k_{uncat}[MB^+][S^{2-}]^3$ with values of $k_{cat}=0.150 s^{-1}$ and $k_{uncat}=0.122 s^{-1}$ with an ionic strength of 0.05M KNO3. The values of activation energies for the Se(lV)-catalysed and uncatalysed reactions were found to be 18.43 and 18.70 kcal.$mol^{-1}$, respectively. 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-determining step in the absence and presence of Se(IV). On the basis of the reaction orders and kinetic salt effect, the reaction mechanism is suggested with and without catalyst.

The reaction between Maxilon blue $(MB^+)$ and the sulfide ion is an ideal kinetic experiment in order to show the dependence of the reaction rate on the concentrations of reactants, ionic strength and temperature. By using excess concentrations of sulfide, the reaction or the disappearance of $(MB^+)$ was monitored spectrophotometrically and kinetically at 654 nm by measuring the decrease in absorbance of $(MB^+)$ . The indicator reaction follows pseudo-first-order kinetics. Specifically, the reduction of $(MB^+)$ with $S^{2-}$ ion in the presence of Se(IV) in an phosphate buffer medium, pH=6.5 was studied. The rate law for this system was found to be $rate_i=k_{cat}[MB^+][S^{2-}]^3[Se(IV)] + k_{uncat}[MB^+][S^{2-}]^3$ with values of $k_{cat}=0.150 s^{-1}$ and $k_{uncat}=0.122 s^{-1}$ with an ionic strength of 0.05M KNO3. The values of activation energies for the Se(lV)-catalysed and uncatalysed reactions were found to be 18.43 and 18.70 kcal.$mol^{-1}$, respectively. 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-determining step in the absence and presence of Se(IV). On the basis of the reaction orders and kinetic salt effect, the reaction mechanism is suggested with and without catalyst.