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.

Iodomalonic acid as an anti-inhibitor in the resorcinol inhibited Briggs-Rauscher reaction.

It was found that the inhibitory effect of resorcinol is less pronounced if it is added in a later stage of the Briggs-Rauscher reaction, which indicates that an accumulating intermediate--most probably iodomalonic acid--can suppress the inhibition. In fact, when iodomalonic acid was added to the reaction mixture, the inhibitory period was shortened considerably even at micromolar levels of the iodomalonic acid concentration. Moreover, iodomalonic acid can accelerate the rate of the reaction when applied in the same low concentrations, suggesting that it can be an autocatalytic intermediate of the Briggs-Rauscher reaction.

Reaction routes leading to CO2 and CO in the Briggs-Rauscher oscillator: analogies between the oscillatory BR and BZ reactions.

With Fenton-type experiments, it is shown that the intense CO2/CO evolution in the Briggs-Rauscher (BR) reaction is due to decarboxylation/decarbonylation of organic free radicals. The metal ion applied in the Fenton-type experiments was Fe2+ or Ti3+ or Mn2+ combined with H2O2 or S2O(8)(2-) as a peroxide, whereas the organic substrate was malonic acid (MA) or a 1:1 mixture of MA and iodomalonic acid (IMA). Experiments with a complete BR system applying MA or the MA/IMA mixture indicate that practically all CO2 and CO comes from IMA. The decarboxylation/decarbonylation mechanisms of various iodomalonyl radicals can be analogous to that of the bromomalonyl radicals studied already in the Belousov-Zhabotinsky (BZ) reaction. It is found that an intense CO2/CO evolution requires the simultaneous presence of H2O2, IO3-, Mn2+, and IMA. It is suggested that the critical first step of this complex reaction takes place in the coordination sphere of Mn2+. That first step can initiate a chain reaction where organic and hydroperoxyl radicals are the chain carriers. A chain reaction was already found in a BZ oscillator as well. Therefore, the analogies between the BR and BZ oscillators are due to the fact that in both mechanisms, free radicals and, in most cases, also transition-metal complexes play an important role.