Prevalence of thermodynamic control in the reaction of thiosulphate ion with 2, 6-dichlorophenolindophenol in phosphate buffer

Abstract

The interaction of thiosulphate ion (TS) with 2, 6-dichlorophenolindophenol (In) has been investigate kinetically in phosphate buffer and tetrathionate has been detected as the oxidation product. The r4action is first order in TS while the order in In changes from unity to two after 35-40% completion of the reaction in a given run. The pseudo-first order rate constant increases with increase in [In] while the second order rate constant decreases under identical conditions. The reaction has an induction period at low In which disappears at higher concentrations. The rate constant increases as the square power of [H⁺] while it decreases on adding acrylonitrile to the reaction system indicating the participation of free radicals in the reaction system. The external addition of tetrathionate and leuco base does not affect the rate. The rate does not alter on varying the ionic strength and the dielectric constant of the medium. In contrast to the observed behaviour, the activation parameters predict the prominence of second order over the first order reaction and the anomaly has been explained in terms of the prevalence of thermodynamic control over kinetic control in the present case.