Wedzicha, Bronislaw Leon (1973)
The kinetics of redox reactions involving thiosulphate.
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The progress of the oxidation of thiosulphate by ferricyanide ion has been followed spectrophotometrically in buffered solution over the pH range 4--7. Tracer studies with 35S showed the presence of polythionate as the major product of oxidation, some sulphate being formed when equivalent concentrations of the reactants were used.The reaction was found to be catalysed by tetrathionate, and kinetics in the absence of products were studied as initial rates. Two rate laws, both independent of ferricyanide, were found to apply. One of these, first order with respect to hydrogen and second order with respect to thiosulphate was shown to proceed by a mechanism which has previously been observed for the initial stages of the acid catalysed decomposition of thiosulphate. No detailed mechanism was proposed for the second contributor which appeared to be dominant at high cation concentration where the order with respect to hydrogen, thiosulphate and cation was one. In the later stages, the catalysed reaction was resolved by fitting rate-[S 4O62--] data to a second degree polynomial in tetrathionate concentration. This expression was found to be equivalent to a rate law of first order with respect to ferricyanide and tetrathionate ions, in agreement with previously reported work.
Kinetic data for the oxidation of thiosulphate by iodate and by bromate ions are presented, the former reaction showing catalysis by potassium and caesium ions but not sodium ion. The latter reaction, for which only a preliminary study was made, showed general acid catalysis. Both processes gave rate laws, with respect to thiosulphate, hydrogen and halate ions, which were the same as those reported by previous authors and the observed results were explained in terms of well established mechanisms.
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Institution: University of London, Bedford College (United Kingdom).