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.

HOI versus HOIO selectivity of a molten-type AgI electrode.

UNLABELLED: AgI electrode is often applied not only to determine iodine concentration but also to follow oscillations in the weakly acidic medium of the Bray-Liebhafsky and Briggs-Rauscher reactions where it partly follows the hypoiodous acid (HOI) concentration. It is known that HOI attacks its matrix in the corrosion reaction: AgI + HOI + H(+) ⇆ Ag(+) + I2 + H2O and the AgI electrode measures the silver ion concentration produced in that reaction. The signal of the electrode can be the basis of sensitive and selective HOI concentration measurements only supposing that an analogous corrosive reaction between AgI and iodous acid (HOIO) can be neglected. To prove that assumption, the authors calibrated a molten-type AgI electrode for I(-), Ag(+), HOI, and HOIO in 1 M sulfuric acid and measured the electrode potential in the disproportionation of HOIO, which is relatively slow in that medium. Measured and simulated electrode potential versus time diagrams showed good agreement, assuming that the electrode potential is determined by the HOI concentration exclusively and the contribution of HOIO is negligible. An independent and more direct experiment was also performed giving the same result. HOIO was produced with a new improved recipe. CONCLUSION: an AgI electrode can be applied to measure the HOI concentration selectively above the so-called solubility limit potential.

Kinetics of the iodate reduction by hydrogen peroxide and relation with the Briggs-Rauscher and Bray-Liebhafsky oscillating reactions.

The iodate reduction by hydrogen peroxide in acidic solutions is part of the Bray-Liebhafsky and Briggs-Rauscher oscillating reactions. At low hydrogen peroxide concentrations, typical of the Bray-Liebhafsky reaction, its rate law is -d[IO(-)(3)]/dt = (k'(R) + k"(R)[H(+)])[IO(-)(3)][H(2)O(2)] with k'(R) = 1.3 × 10(-7)(20°), 7.8 × 10(-7) (39°), 1.4 × 10(-5) M(-1) s(-1) (60°) and k"(R) = 1.5 × 10(-5) (25°), 6.2 × 10(-5) (39°), 6.3 × 10(-4) M(-2) s(-1) (60°). It is explained by a non-radical mechanism. At high hydrogen peroxide concentrations, typical of the Briggs-Rauscher reaction, a new reaction pathway appears with a rate more than proportional to [H(2)O(2)](2) and nearly independent of [IO(3)(-)] > 0.01 M. This pathway is inhibited by scavengers of free radicals. We suggest that it has a radical mechanism starting with IOH + H(2)O(2)⇌ IOOH + H(2)O and IOOH+H(2)O(2)→ IO˙ + H(2)O+HOO˙.

Iodine oxidation by hydrogen peroxide and Bray-Liebhafsky oscillating reaction: effect of the temperature.

This work presents a new experimental kinetic study at 39° and 50° of the iodine oxidation by hydrogen peroxide. The results allow us to obtain the temperature effect on the rate constants previously proposed at 25° for our model of the Bray-Liebhafsky oscillating reaction (G. Schmitz, Phys. Chem. Chem. Phys. 2010, 12, 6605.). The values calculated with the model are in good agreement with many experimental results obtained under very different experimental conditions. Numerical simulations of the oscillations observed formerly by different authors are presented, including the evolutions of the iodine, hydrogen peroxide, iodide ions and oxygen concentrations. Special attention is paid to the perturbing effects of oxygen and of the iodine loss to the gas phase.

Iodine oxidation by hydrogen peroxide in acidic solutions, Bray-Liebhafsky reaction and other related reactions.

The kinetics of the iodine oxidation by hydrogen peroxide is a complicated function of the concentrations of iodine, hydrogen peroxide, perchloric acid and iodate. A proposed model in eight steps explains the new experimental results. It explains also the effect of the concentrations at the steady state of the hydrogen peroxide decomposition catalyzed by iodine and iodate. Without iodate added initially, the iodine oxidation by hydrogen peroxide is preceded by an induction period that depends on the oxygen concentration. A simple extension of the proposed model gives a semi-quantitative explanation of the oxygen effect and allows simulations of the Bray-Liebhafsky oscillations at 25 degrees C.

Stoichiometric network analysis and associated dimensionless kinetic equations. Application to a model of the Bray-Liebhafsky reaction.

The stoichiometric network analysis (SNA) introduced by B. L. Clarke is applied to a simplified model of the complex oscillating Bray-Liebhafsky reaction under batch conditions, which was not examined by this method earlier. This powerful method for the analysis of steady-states stability is also used to transform the classical differential equations into dimensionless equations. This transformation is easy and leads to a form of the equations combining the advantages of classical dimensionless equations with the advantages of the SNA. The used dimensionless parameters have orders of magnitude given by the experimental information about concentrations and currents. This simplifies greatly the study of the slow manifold and shows which parameters are essential for controlling its shape and consequently have an important influence on the trajectories. The effectiveness of these equations is illustrated on two examples: the study of the bifurcations points and a simple sensitivity analysis, different from the classical one, more based on the chemistry of the studied system.

Complex and chaotic oscillations in a model for the catalytic hydrogen peroxide decomposition under open reactor conditions.

Numerous periodic and aperiodic dynamic states obtained in a model for hydrogen peroxide decomposition in the presence of iodate and hydrogen ions (the Bray-Liebhafsky reaction) realized in an open reactor (CSTR), where the flow rate was the control parameter, have been investigated numerically. Between two Hopf bifurcation points, different simple and complex oscillations and different routes to chaos were observed. In the region of the mixed-mode evolution of the system, the transitions between two successive mixed-mode simple states are realized by period-doubling of the initial state leading to a chaotic window in which the next dynamic state emerges mixed with the initial one. It appears in increasing proportions in concatenated patterns until total domination. Thus, with increasing flow rate the period-doubling route to chaos was obtained, whereas with decreasing flow rate the peak-adding route to chaos was obtained. Moreover, in very narrow regions of flow rates, chaotic mixtures of mixed-mode patterns were observed. This evolution of patterns repeats until the end of the mixed-mode region at high flow rates that corresponds to chaotic mixtures of one large and many small amplitude oscillations. Starting from the reverse Hopf bifurcation point and decreasing the flow rate, simple small amplitude sinusoidal oscillations were encountered and then the period-doubling route to chaos. With a further decreasing flow rate, the mixed-mode oscillations emerge inside the chaotic window.