THE KINETICS OF THE FERRIC THIOSULPHATE REACTION. PART I

From the mechanism involving the two intermediates FeS2O3+ and Fe (S2O3)2-, suggested for the ferric thiosulphate reaction (Patnaik and Mahapatra, Curr. Sci., 1955, 25, 195), kinetic equations have been derived for the initial and maximum rates of decay which have been measured spectrophotometri­cally at different ionic strengths for varying concentrations of ferric and thiosulphate ions. The initial rate varies as the first power of the concentrations of the ferric ion and the square of the concentration of the thiosulphate ion. On the other hand, the maximum rate varies as the square of the concentration of the coloured mixed complex FeS2O3+. These observations conform to the kinetic equations derived for the rates from the proposed mechanism, the various steps of which consist of bimolecular reactions between oppositely charged ions and ion pairs. The decrease in rates; both initial and maxima with the increase in ionic strength substantiates the fact that the rate-controlling stages are reactions involving oppositely charged ions