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.

Measurement of hypoiodous acid concentration by a novel type iodide selective electrode and a new method to prepare HOI. Monitoring HOI levels in the Briggs-Rauscher oscillatory reaction.

A new type of iodide selective electrode prepared by dipping a silver wire into molten silver iodide is reported. The electrode was calibrated for silver and iodide ions and the measured electromotive force for various Ag(+) and I(-) concentrations was close to the theoretical within a few millivolts. Besides Ag(+) and I(-) ions, however, the electrode also responds to hypoiodous acid. Thus, the electrode was calibrated for HOI as well, and for that purpose a new method of hypoiodous acid preparation was developed. To explain the close to Nernstian electrode response for HOI and also the effect of hydrogen ion and iodine concentration on that response, the corrosion potential theory suggested earlier by Noszticzius et al. was modified and developed further. Following oscillations in the Briggs-Rauscher reaction with the new electrode the potential crosses the "solubility limit potential" (SLP) of silver iodide. Potentials below SLP are controlled by the concentration of I(-), but potentials above SLP are corrosion potentials determined by the concentration of HOI. Finally, the measured HOI oscillations are compared with calculated ones simulated by a model by Furrow et al.

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˙.

Experimental and mechanistic investigation of an iodomalonic acid-based Briggs-Rauscher oscillator and its perturbations by resorcinol.

Classic Briggs-Rauscher oscillators use malonic acid (MA) as a substrate. The first organic product is iodomalonic acid. Iodomalonic acid (IMA) can serve as a substrate also; thus, the first product in that case is diiodomalonic acid (I(2)MA). Nonoscillating iodination kinetics can be followed by absorbance at 462 nm in acidic KIO(3) so long as IMA is in substantial excess over [I(2)]. At 25 °C, simulations lead to the two most important rate laws, and related rate constant estimates are reported. I(2)MA eventually decomposes by unknown processes, but I(2), O(2), H(2)O(2), and Mn(2+) speed up that decomposition, liberating most of the iodine back to the solution. Resorcinol is an effective inhibitor of oscillations both in MA oscillators and in IMA oscillators. Response of an IMA oscillator to varying amounts of resorcinol is shown herein and is similar to that for MA-based oscillators. The inhibitory effect of resorcinol is diminished by addition of IMA to a MA-based oscillator. The iodination reaction between IMA and resorcinol is too slow (0.043 M(-1) s(-1)) to account for the decreased inhibitory effectiveness of resorcinol. Rather, the decomposition of I(2)MA is responsible for the inhibition decrease.

Reactions of iodomalonic acid, diiodomalonic acid, and other organics in the Briggs-Rauscher oscillating system.

It was found that oxalic acid is one of the main products in the Briggs-Rauscher oscillating reaction. In nonoscillating solutions, oxidation of iodomalonic acid and/or diiodomalonic acid by Fenton-type reactions also produced oxalic acid as well as I(2). Mesoxalic acid yielded oxalic acid under similar conditions. Tartronic acid was nearly inert to Fenton-type reactions; however, tartronic acid was oxidized by iodate and iodine to mesoxalic acid, which in turn could form oxalic acid in the presence of H(2)O(2) plus catalyst. Iodotartronic acid appeared to be a short-lived but significant intermediate, thus both tartronic acid and mesoxalic acid are possible intermediates. Glycolic acid and glyoxylic acid are not intermediates in the oxidation of iodomalonic acid, since they in turn produce formic acid under similar nonoscillating conditions.