Reexamination of gas production in the Bray-Liebhafsky reaction: what happened to O2 pulses?

Experimental support for the model of Bray-Liebhafsky oscillatory reaction based on the heterogeneous effects

Recently, we developed a conceptually new model of the Bray-Liebhafsky (BL) oscillatory reaction based on specific redistribution of energy connected with the formation of a new gaseous phase in the system. The model predicts that the amplitude of concentration oscillations and their frequency are tightly connected with the total amount of oxygen in the solution, present in the form of dissolved molecules and oxygen bubbles (gas cavities). As a new conceptual approach, it needs extended experimental support for further corroboration. In this work, we developed an experimental setup for simultaneous measurement of the redox state of the system by a Pt electrode, flow of the oxygen above the reaction mixture and temperature of the solution. In this way the oscillatory evolution is correlated with the formed oxygen in the system at different mixing rates and the highly exothermic development of the oxidation branch. The obtained results give strong support for the predictions of the model and offer a new conceptual base for deeper understanding of the reaction mechanism.

Platinum as a HOI/I2 Redox Electrode and Its Mixed Potential in the Oscillatory Briggs-Rauscher Reaction

Pt is a common redox electrode used to follow oscillations qualitatively in the Briggs-Rauscher (BR) and the Bray-Liebhafsky (BL) reactions from the time of their discovery. Although the potential oscillations of the electrode reflect the temporal pattern of the reaction properly, there is no general agreement as to how that potential is determined by the components of the reaction mixture. In this article, first we investigate how iodine species in different oxidation states affect the potential of a Pt electrode. It was found that I(+3) and I(+5) species do not affect the potential; only I^-, I₂, and HOI may have an influence. Although the latter three species are always present simultaneously as participants of the rapid iodine hydrolysis equilibrium, it was found that below and above the so-called hydrolysis limit potential (HLP, where the iodide and HOI concentrations are equal) the actual potential determining redox couple is different. Below the HLP, it is the traditional I₂/I^- redox couple, but above the HLP, it is the HOI/I₂ redox pair that determines the potential of a Pt electrode. That change in the potential control mechanism was proven experimentally by exchange current measurements. In addition, from the potential response of the Pt electrode below and above the HLP, it was possible to calculate the equilibrium constant of the iodine hydrolysis as K°_H = (4.97 ± 0.20) × 10^-13 M², in rather good agreement with earlier measurements. We also studied the perturbing effect of H₂O₂ on the previously mentioned potentials. The concentration of H₂O₂ was 0.66 M, as in the BR reaction studied here. It was found that below the HLP, the perturbing effect of H₂O₂ was minimal but above the HLP, H₂O₂ shifted the mixed potential considerably down toward the HLP. In our experiments with the BR reaction, the potential oscillations of the Pt electrode crossed the HLP, indicating that from time to time the HOI concentration exceeds that of the iodide. We can conclude that although the perturbing effect of H₂O₂ prevents the calculation of concentrations from Pt potentials above the HLP, [I^-]/[I₂]^1/2 ratios can be calculated as a good approximation from Pt potentials below the HLP.

Intermittent chaos in the Bray-Liebhafsky oscillator. Temperature dependence

Intermittent oscillations as a chaotic mixture of large amplitude relaxation oscillations, grouped in bursts and small-amplitude sinusoidal ones or even quiescent parts between them known as gaps, were found and examined in the Bray-Liebhafsky (BL) reaction performed in CSTR under controlled temperature variations. They were obtained in a narrow temperature range from 61.0 °C to 63.1 °C, where 61.0 °C is the critical temperature for burst emergence from the stable steady state and 63.1 °C is the critical temperature for gap emergence from regular oscillations. Since intermittencies appear gradually from the regular oscillatory state, and no hysteresis was obtained with decreasing/increasing temperature in the vicinity of these two bifurcations, a linear relationship between (τB/τ)(2) and (τG/τ)(2) (where τB, τG and τ denotes duration of bursts, gaps, and whole experiment, respectively), as a function of the temperature as the control parameter, was expected and obtained. Although these intermittent oscillations are chaotic with respect to the lengths of individual gaps as well as bursts, their deterministic behavior related to temperature was additionally established. Thus, the number of bursts or gaps per unit of time (NB/τ and NG/τ) has the form of a normal distribution function over the temperature range in the region where intermittencies are obtained. Temperature dependence of the Lyapunov exponents was also described by a function of the normal distribution form. Hence, we established some regularities in the chaotic behavior of intermittent oscillations that are common in life but difficult for determinations.