Kinetics and mechanism of the reaction between hypochlorous acid and tetrathionate ion

The reaction has been studied spectrophotometrically monitoring the absorba nce in the 240-400 nm wavelength range. The spectra of the reactants, inter mediates, and products in this system are overlapping; thus special program s [1,2] have been used (and tested) to unravel the kinetics and mechanism o f the reaction. The stoichiometry of the reaction in excess hypochlorous ac id is S4O62- + 7HOCl + 3H(2)O --> 4SO(4)(2-) + 7Cl(-) + 13H(+). Various exp eriments are presented to show that--in excess tetrathionate-the reaction p roduces a light-absorbing intermediate identified as S2O3Cl-. The intermedi ate slowly hydrolyzes into sulfur and sulfate and it yields pentathionate i n excess tetrathionate. The rate-determining steps and their rate constants are S4O62- + HOCl k(1) / --> S2O3Cl- + S2O42- + H+; k(1) = 32.4 M(-1)s(-1) S2O3Cl- + HOCl + 2H(2)O k(2) / --> 2SO(3)(2-) + 2Cl(-) + 5H(+); k(2) = 7.7 M(-1)s(-1) The further oxidation of S2O42- and SO32- by HOCl to sulfateare fast proces ses. (C) 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 395-402, 2000.