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.

CO and CO2 evolution was measured in a cerium and in a ferroin-catalyzed Belousov-Zhabotinsky (BZ) reaction. These gases were stripped from the reaction mixture by a N2 carrier gas, mixed with H2, converted to methane on a Ni catalyst, and then measured by a flame ionization detector (FID). CO could be detected separately by absorbing CO2 on a soda lime column. In separate experiments it was proven that CO is produced in a reaction of BrO2* radicals with bromomalonic acid (BrMA). To this end BrO2(.-) radicals were generated in two different ways: (i) in the reaction HBrO2 + HBrO3 <--> 2 BrO2(.-) + H2O and (ii) by reducing HBrO3 to BrO2(.-) by Fe(2+). It was found that (.-)OH radicals--produced by Fenton's reagent--can also generate CO from BrMA. We propose that CO can be formed when an inorganic radical (like BrO2(.-) or (.-)OH) reacts with the enol form of BrMA producing an acyl radical which decarbonylates in the next step. Malonic acid (MA)-BrMA mixtures were prepared by a new method modifying Zaikin and Zhabotinsky's original recipe to minimize the production of dibromomalonic acid (Br2MA).

This study is initiated by a recent discovery, according to which a water soluble polymer, poly(ethylene glycol) (PEG), affects the dynamics of the Belousov-Zhabotinsky (BZ) reaction in a characteristic way. As various polymers and polymer-based hydrogels are often applied in nonlinear chemical experiments, it is an interesting question whether the effect of a polymer can be attributed exclusively to its reactive endgroups (here primary alcoholic groups) or if the macromolecular nature of the perturbant might be also important. In this paper, as a first step, the results of batch experiments are presented applying only small molecules, namely ethylene glycol (the monomer of PEG) and methanol (a more simple primary alcohol), as perturbants of the BZ reaction. The reaction was followed by monitoring the rate of the carbon dioxide evolution. The experimental results are compared with model calculations, applying the latest model of the BZ reaction, the Marburg-Budapest-Missoula (MBM) mechanism extended with the perturbing reactions. The rate of the perturbing reactions (reaction of the acidic bromate with the alcohol producing the autocatalytic intermediate bromous acid) was determined in separate spectrophotometric experiments. Experiments and model calculations show a good qualitative agreement (alcoholic perturbations increase the induction period and the frequency of the oscillations and decrease the amplitude), but disagreements were found on a quantitative level. Because the mechanism of the alcoholic perturbation, especially in the case of methanol, is mostly clarified, it is the MBM mechanism which should be modified somewhat in the future. As the reaction dynamics responds to the alcoholic perturbations rather sensitively, simulating these perturbation experiments can help to test new mechanistic proposals for the BZ reaction.

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