Fluctuations and stirring rate effects in the chlorite-thiosulfate reaction

Abstract

The stoichiometry, kinetics, and mechanism of the reaction between chlorite and thiosulfate have been studied at 25°C and pH 6-9. In excess thiosulfate, the stoichiometry is 4S2O32- + ClO2- + 2H2O = 2S4O62- + 4OH- + Cl-. In excess chlorite, sulfate is produced as well, the stoichiometry being a mixture of the above reaction and S2O3 + 2ClO2- + H2O = 2SO42- + 2Cl- + 2H+. The rate of production of OH- is d(δ[OH-])/dt = kexpt[S2O32-] [ClO2-][H+], but the rate constant kexpt varies with the [ClO2-]/[S2O32-] ratio, being about twice as large in excess S2O32- as in excess ClO2-. A mechanism is proposed involving the complex intermediates S2O3ClO- and S2O3Cl-, a key reaction between the simpler intermediates SO32- and ClO- and the "supercatalytic" chlorite-tetrathionate reaction. At pH ≈ 11 in unbuffered solution, the reaction behaves as a "clock" reaction, with an initial rise in pH followed by an abrupt drop. The reaction time, however, varies irreproducibly, even in identically prepared samples. Careful analysis of the reaction time distribution and its variation with temperature, volume reactant concentrations, and stirring rate leads to the conclusion that the switch from net OH- production to net H+ generation is induced by random fluctuations within the solution. The implications of this interpretation are discussed. © 1986 American Chemical Society.