Further evidence for radical-controlled oscillations in the Belousov-Zhabotinskii reaction: large effects of ultraviolet light and silver ions (original) (raw)
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The role of radicals in the Belousov-Zhabotinsky reaction
Reaction Kinetics and Catalysis Letters, 1990
A new theory of the Belousov-Zhabotinsky (BZ) reaction, the Radicalator model, is presented. This model is based on a negative feedback loop involving a fast reaction between malonyl and bromine dioxide radicals. Experimental evidence for the validity of the model is given for BZ systems in 3 M and 1 M sulfuric acid solution.
The Journal of Physical Chemistry A, 1998
New experimental results on the oscillatory dynamics of the radical-controlled Belousov-Zhabotinsky reaction (the Rácz system) in a batch reactor are reported. The system exhibits oscillations with no induction period, a typical feature of the radical-controlled mechanism. However, in the presence of acetylacetone (CH 2-(COCH 3) 2), an induction period is observed before oscillations start, which increases with increasing acetylacetone concentration. There is a critical concentration of acetylacetone at which no oscillations occur. Quenching of radical-controlled oscillations is also observed at low and high malonic acid concentrations as well as at low and high sulfuric acid concentrations. An induction period is observed before the onset of radical-controlled oscillations at sulfuric acid concentrations g5.5 M. The duration of radical-controlled oscillations reaches a maximum at an intermediate sulfuric acid concentration. Numerical simulations based on the Radicalator model predict limits of malonic acid and sulfuric acid concentration within which oscillations are observed. The Radicalator model with additional reactions involving (i) CH 2 (COCH 3) 2 + Ce 4+ f • CH-(COCH 3) 2 , (ii) • CH(COCH 3) 2 + BrO 2 • f products, and (iii) • CH(COCH 3) 2 + • CH(COCH 3) 2 f products also predicts lengthening of induction period with the increase of acetylacetone concentration and suppression of oscillations at high acetylacetone concentration. Inclusion of the reaction between acetylacetone and HOBr had no effect.
Mechanistic details of the oscillatory Belousov-Zhabotinskii reaction
The Journal of Physical Chemistry, 1990
The reactions constituting the mechanism of the oscillatory Belousov-Zhabotinskii (BZ) reaction may be divided into an inorganic and an organic subset. The former is well established and generally accepted, but the latter remains under development. There has been considerable work on component reactions of the organic subset over the past few years, but little effort has been made to incorporate the results of this work into an improved BZ mechanism. We do so and present a BZ mechanism containing 80 elementary reactions and 26 variable species concentrations and which implements recent experimental results and suggestions concerning the complicated organic chemistry involved. The possible role of organic radicals as a second control intermediate is explored. The rate constants of the inorganic subset also are adjusted for acidity effects. The performance of the model in simulating either quantitatively or semiquantitatively a number of recent BZ experiments is substantially better than that of previous models. Several areas in need of further work are identified. ( I I ) Ganapathisubramanian, N.; Noyes, R. M. J . Phys. Chem. 1982,86, (12) Noszticzius, 2.; Gispiir, V.; Forsterling, H.-D. J . Am. Chem. SOC. (13) Brusa, M. A.; Perissinotti, L. J.; Colussi, A. J.
The Journal of Physical Chemistry A, 2004
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.
Physical Chemistry Chemical Physics, 2000
High-pressure liquid chromatography (HPLC) and measurements of the produced were performed in the CO 2 induction period of the classical BelousovÈZhabotinsky (BZ) reaction (malonic acidÈbromateÈcerium catalyst in sulfuric acid medium). It was found that oxalic acid is a Ñow-through intermediate of the reaction. This was conÐrmed with an independent qualitative test with thiobarbituric acid. The concentration of oxalic acid grows in the induction period together with that of bromomalonic acid and dibromomalonic acid intermediates. It is known that there are two negative feedback loops in the BZ reaction : one is via bromide and the other via organic free radicals. Oxalic acid and also are products of this second loop where CO 2 organic radicals react with radicals. The induction period was chosen for the present experimental BrO 2 studies because the above radicalÈradical reactions are most intense during that time. Based on the experimental results mechanistic proposals are made for the radical feedback loop. A method to accumulate multivalent organic acids present in very low concentrations in the BZ reaction was also developed. Applying this and a thermal decomposition method ethenetetracarboxylic acid (EETA) was identiÐed as an oxidation product of ethanetetracarboxylic acid (ETA).
The Journal of Physical Chemistry, 1986
The rate constant of the disproportionation of HBr02, 2HBr02-HBr03 + HOBr, an important step of the Belousov-Zhabotinskii oscillating system, was measured spectrophotometrically at 240 nm by using stopped-flow techniques. Its value at [HzS04] = 0.5 M, T = 24 OC was found to be k4 = 2.2 X lo3 M-l. R ough measurements with bromide-selective electrodes led to comparable results. This value is 4 orders of magnitude smaller than the one given by Field, Koros, and Noyes. Consequently, many other rate constants, whose values are known only relative to k4, will have to be changed as well.
Controversial interpretations of silver(1+) perturbation of the Belousov-Zhabotinskii reaction
The Journal of Physical Chemistry, 1989
Addition of silver ion to a Belousov-Zhabotinsky (BZ) system affects the rate at which bromide ion is being removed and thereby influences the observed processes. Noszticzius and McCormick on the basis of potentiometric measurements have concluded that the precipitation of AgBr can be described with a second-order rate constant of about lo9 M-' s-', corresponding to virtual diffusion control. Varga and Kcirb and also Ruoff have used more indirect methods and modeling computations to conclude that the rate constant for the same process is approximately lo4 M-I s-'. The resulting disagreement may be resolved by the spectrophotometric studies of the precipitation by Kshirsagar, Field, and Gycirgyi. These authors conclude that Ag+ and Br-react very rapidly homogeneously to form complex ions and small oligomers of AgBr; these reactions rapidly reduce the activities of silver and of bromide ions as shown by the potentiometric studies. However, bromide ion in or from these complexes is 'still available for reaction with HBrOz in the BZ reaction. The subsequent precipitation of AgBr on the surface of growing crystals is a much slower process consistent with the other measurements. This resolution of the disagreement is supported by model computations that indicate that silver-induced oscillations are still controlled by free or lightly complexed bromide ion much as are other Belousov-Zhabotinsky oscillations.