PREVALENCE OF THERMODYNAMIC CONTROL IN THE REACTION OF THIOSULFATE ION WITH 2,6-DICHLOROPHENOLINDOPHENOL IN PHOSPHATE BUFFER

The interaction of thiosulphate ion (TS) with 2, 6-dichlorophenolindop henol (In) has been investigated kinetically in phosphate buffer and t etrathionate has been detected as the oxidation product. The reaction is first order in TS while the order in In changes from unity to two a fter 35-40% completion of the reaction in a given run. The pseudo-firs t order rate constant increases with increase in [In] while the second order rate constant decreases under identical conditions. The reactio n has an induction period at low [In] which disappears at higher conce ntrations. The rate constant increases as the square power of [H+] whi le it decreases on adding acrylonitrile to the reaction system indicat ing the participation of free radicals in the reaction system. The ext ernal addition of tetrathionate and leuco base does not affect the rat e. The rate does not alter on varying the ionic strength and the diele ctric constant of the medium. In contrast to the observed behaviour, t he activation parameters predict the prominence of second order over t he first order reaction and the anomaly has been explained in terms of the prevalence of thermodynamic control over kinetic control in the p resent case.