This paper is the first part of a study reinvestigating the mechanism of the Belousov−Zhabotinsky (BZ) reaction of oxalic acid, which is the simplest organic substrate for a BZ oscillator. New experiments are performed to find the oscillatory region in 1 M sulfuric acid at 20 °C. The removal rate of the end product bromine by an inert gas stream is a critical parameter here: oscillations can be observed only in a window of that parameter. The “rate constant” for the physical removal of bromine is measured as a function of the gas flow rate and reactor volume; furthermore, the rate constants of three component reactions important in this system are also determined. These are oxygen atom transfer reactions to the oxalic acid substrate from Br(I) (hypobromous acid), from Br(III) (bromous acid), and from Br(V) (acidic bromate) compounds. In these second-order reactions, the partial order of each oxybromine species is 1. The measured rate constants are k I = 17 ± 2 M-1 s-1, k III = 4.2 ± 0.5 M-1 s-1, and k V = (7.47 ± 0.1) × 10-4 M-1 s-1. In the case of the HOBr−oxalic acid reaction, however, an additional parallel reaction route was found that has importance at higher HOBr concentrations. In the mechanism of that new route, the active species is Br2O, and the reaction order is not 1 but 2 with respect to HOBr. The rate constant of this parallel reaction is k I (2) = (1.2 ± 0.2) × 105 M-2 s-1. The k values measured here are compared with those reported earlier. A comparison of experimental results with computer simulations shows that free radicals play a negligible role or no role in the mechanism of the oxygen atom transfer reactions studied here.
The aim of the present paper is to study radical reactions important in the mechanism of the Belousov-Zhabotinsky (BZ) reaction with its simplest organic substrate, oxalic acid, and to model the oscillatory system applying the newly determined rate constants. We considered five radical species in this BZ system: carboxyl radical, bromine atom, dibromine radical ion, and bromine monoxide and dioxide radicals. To study separately reactions of only three radicals, • CO 2 H, • Br, and • Br 2 -, semibatch experiments were performed. The semibatch reactor contained oxalic acid, elemental bromine, and bromide ions in a solution of 1 M H 2 SO 4 at 20 °C, and a continuous inflow of Ce 4+ generated carboxyl radicals. The carboxyl radicals initiate a chain reaction: first they react with elemental bromine and produce bromine atoms (CR1); then the bromine atoms react with oxalic acid, producing carboxyl radicals again (CR2). Consumption of elemental bromine in the chain reaction was followed with a bright Pt electrode. By measuring the stoichiometry of the chain reaction, it was possible to determine or estimate several rate constants. It was found that CR1 is a fast reaction with an estimated k value of more than 10 9 M -1 s -1 . The rate constant of CR2 is 7 × 10 5 M -1 s -1 , and the k value for the Ce 4+ -• CO 2 H reaction is 1.5 × 10 9 . These values were obtained by comparing experiments with model calculations. Such simulations also suggested that a reaction of • Br 2with oxalic acid, analogous to CR2, plays a negligible role or no role here. Simulations of the oscillatory system applied rate constants, which were known from the literature, or determined here or in the first part of our work, and some unknown rate constants were estimated on the basis of analogous radical reactions. To obtain an optimal fit between experiments and simulations, only one rate constant was used as a variable parameter. This was the reaction of carboxyl radical with acidic bromate with an optimal k value of 1.0 × 10 7 M -1 s -1 . Agreement between experimental and simulated oscillations was satisfactory at low bromine removal rates (that rate was controlled by a nitrogen gas flow), but a disagreement was found at higher flow rates. Possible reasons for this disagreement are discussed in the conclusion.
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