Oscillatory Reactions Involving Hydrogen Peroxide and ThiosulfateKinetics of the Oxidation of Tetrathionate by Hydrogen Peroxide

Abstract

The reaction between tetrathionate and hydrogen peroxide forms an important part of several pH oscillators based on the oxidation of thiosulfate. The kinetics of this reaction were examined in a batch reactor by measurement of the initial pH values in the range from 8 to 10.5. Experimental data were evaluated by the method of initial reaction rates combined with the assumption of instantaneously equilibrated acid-base reactions. The rate-determining step was found to be of the first order with respect to tetrathionate, hydrogen peroxide, and OH- ions with the value of rate constant k = (1.50 +/- 0.03) x 10(2) (M2 s)(-1) at 25 degrees C. In the alkaline solution, no distinct catalytic effect of Cu2+ was observed in contrast to earlier assumptions. The waveform of measured pH over the course of the reaction indicates that thiosulfate is probably an intermediate because characteristics of the curves are very similar to those for the oxidation of thiosulfate. We also measured the time evolution of concentrations of major components by the attenuated total internal reflectance infrared spectroscopy to further elucidate the underlying reaction mechanism. These measurements confirm the suspected role of thiosulfate as an intermediate in the studied reaction and provide valuable detailed information on the time evolution of thiosulfate, sulfite, sulfate, tetrathionate, and trithionate. These experimental observations are included in a simple mechanism that accurately simulates the reaction course in an alkaline solution. The results provide considerable new insights into the nature of autocatalysis in the hydrogen peroxide-thiosulfate-Cu2+ reaction and suggest that a new role for Cu2+ in the oscillatory dynamics observed in a flow-through reactor needs to be found.

The Importance of Thiosalts Speciation: Review of Analytical Methods, Kinetics, and Treatment

“…In the case of basic environments, Voslar et al (2006) identified thiosulfate as a key intermediate in tetrathionate oxidation by hydrogen peroxide according to the following reactions: …”

“…Sulfite can further being oxidized according to the conditions of the solution and could generate another sulfur intermediate such as trithionate. The reaction rate reported by Voslar et al (2006) is…”

Use of Phase Transition Curves for Testing Models of the pH-Oscillatory Hydrogen Peroxide-Thiosulfate-Sulfite Reaction

“…All steps except R2 are used from the previous mechanism but, in addition, some reactions from the mechanism proposed for the hydrogen peroxide-thiosulfate reaction combined with reactions involving various polythionates, disulfite and disulfurtrioxide considered. The extended mechanism is as follows (for the corresponding rate laws see Table ): …”

On the Phase Shifts in the Dynamics of the H2O2–Na2S2O3–H2SO4–CuSO4 Oscillator Revealed by Comparison of Potentiometric Responses of Different Indicator Electrodes

“…As a consequence of these findings for the hydrogen peroxide–thiocyanate oscillator, we decided to investigate whether similar differences in the potentiometric responses occur also for other homogeneous oscillatory processes, especially for those which involve similar reactants. An example is the H 2 O 2 – S 2 O 3 2− – H + – Cu 2+ system, discovered by Orbán and Epstein and studied also later , including its recent interesting application as a driver for the ribozyme activity . This system also contains hydrogen peroxide as the oxidant for another sulfur‐containing compound and requires the catalytic copper(II)/copper(I) couple.…”