The kinetics and mechanism of the oxidation of inorganic oxysulfur compounds by potassium ferrate part II - Tetrathionate ions

The kinetics of the oxidation of tetrathionate ions by potassium ferrate were investigated under pseudo first-order conditions over the pH range 7.5 to 10.6. At pH values above approximately 8.6 the rate of reaction was first-order in the concentration of ferrate and tetrathionate ions, but inversely related to the hydrogen ion concentration. At lower pH values the rate was approximately first-order with respect to the ferrate ions. The rate increased slightly with the hydrogen ion concentration but was independent of the tetrathionate ion concentration. A possible mechanism at higher pH values involves a rate-determining step between unprotonated ferrate ions and unprotonated tetrathionate ions. At lower pH values it is possible that the deprotonation of singly protonated ferrate ions is the rate-determining step. The rate constant for the rate-determining step at higher pH values was calculated to be 1.2 x 10(2) M-1 s(-1), and at lower pH values it was 8.9 x 10(-3) s(-1). The reaction is relatively slow, taking from 30 to 2000s for completion. The hydrolysis of the tetrathionate ions under reaction conditions was also studied and found not to affect the results.