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We present a new model for the oscillatory peroxidase-oxidase reaction, which is an extension of the recent model C (Aguda, B. D.; Larter, R. J. Am. Chem. SOC. 1991, 113, 7913) and includes reaction steps involving 2,4-dichlorophenol and termination reactions for the superoxide radical and hydrogen peroxide. The calculations with this model compare well with measurements of temporal variations, including mixed-mode, chaotic, and bursting oscillations as well as tests for the assignment of roles of the species in the model and the categorization of the oscillator (Eiswirth, M.; Freund, A.; Ross, J. Adv. Chem. Phys. 1991, 80, 127). Of the 12 species considered in this model, six are essential, and the others nonessential. The essential species are oxygen, compound I, compound 11, compound 111, the NAD radical, and the superoxide radical. The nature of these essential species is also identified. Of the four species (oxygen, NADH, native HRP, and compound 111) measured experimentally, oxygen and compound I11 have been determined to be essential. The category of this model is lCW, in accordance with the experimental results. The new model reproduces better than the earlier models the temporal variations observed experimentally.

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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.