Formation of BrO₂ in the Belousov-Zhabotinsky-System — Investigation of the HBrO₂ /BrO₃ ^–-Reaction

Abstract

The reaction between HBr02 and bromate was studied in sulfuric acid solution. From the spectral dependence of the absorbance in the region from 400 to 600 nm it is concluded that Br02 is an intermediate of the reaction. From the maximum concentration of Br02 the equilibrium constant of the BrOa/B^O-i-equilibrium is evaluated; furthermore, a value for the ratio of the rate constants for the HBr02/Br03~-reaction and the disproportionation reaction of HBr02 is given.The autocatalytic formation of Ce 4+ is one of the essential reactions in the Belousov-Zhabotinskysystem [1,2] In preceding experiments [4,5, 6] we have been able to detect Br02 formed in this reaction. The kinetics experimentally observed was found to be in qualitative agreement with the FKN-reaction scheme. In a quantitative respect, however, the experimental results disagreed with theory; as an example, the Br02-concentrations experimentally obtained were 1/10 of the calculated values only. In order to overcome this discrepany, reaction (R2) was investigated in more detail. Experimental ProcedureSince HBr02 is not stable in acid solution, we have studied reaction (R2) by injecting NaBr02 (solved in 0.1 m NaOH) into aim solution of NaBr(>3 in 1 m sulfuric acid assuming that the reactionis much faster than all other reactions under consideration. Care was taken to use reagents of high purity. 1 m H2SO4 was prepared by dilution of conc. H2S04 (p.a. Merck) in bidistilled water. NaBrÜ3 (p.a. Fluka) was twice recrystallized from hot water. The absorption spectrum of solutions of NaBrC>3 in 1 m H2S04 changed significantly in the course of purification (Fig. 1); this fact is mainly due to the elimination of Br~-and BrO~-impurities. We were not able to reduce the residual 480 nm-absorption band by further recrystallization. NaBr02 was prepared by a modified procedure [7] basing on the synthesis described by Breiss [8]. The product was three times recrystallized from 0.1 m NaOH; the absorption spectrum given in Fig. 2 was found to be in excellent agreement with the spectra reported in [8,9]. The residual Br~-impurity of this product was estimated using the following procedure: alkaline solutions of NaBr02 (initial concentration 2.81 • 10~4 m) were injected into 1 m sulfuric acid and the absorption spectra of the reaction products were taken (Figure 3). In the case of curve 1, NaBr02 was used without further purification; in the case of curve 2, the alkaline solution of NaBr02 had been purified from Br~ by adding silver nitrate and removing the AgBr-precipitate by ultracentrifugation. The difference spectrum of curves 1 and 2 was found to agree with the spectrum of Br2 (curve 3). From the absorbance A =0.018 at X = 400 nm the Br2-concentration 1.1 • 10 -5 m was calculated (fiBr2 = 172 1 • mole-1 • cm-1 ).Assuming the reaction path 3 Br-+ HBr02 + 3 H+ -» 2 Br2 + 2 H20 the concentration of the Br~-impurity was estimat-0340-4811 I 80 / 1200-1354 $ 01.00/0. -Please order a reprint rather than making your own copy.

Bromite Ion Catalysis of the Disproportionation of Chlorine Dioxide with Nucleophile Assistance of Electron-Transfer Reactions between ClO2 and BrO2 in Basic Solution

“…The first step (eq 2) is a thermodynamically uphill electron-transfer reaction to give ClO 2 - and BrO 2 . Bromine dioxide is a highly reactive compound that is capable of disproportionating to form BrO 2 - and BrO 3 - . − However, this does not take place because the concentration of BrO 2 is very low compared to ClO 2 so that the reactions in eqs 3 and 4 generate ClO 3 - and regenerate BrO 2 - . The reaction between BrO 2 and ClO 2 has been proposed as an intermediate step in the reduction of bromate ion by chlorous acid. , The rate constant for the BrO 2 /ClO 2 reaction at 1.0−2.0 M [H + ] has been estimated to be 1.2 × 10 5 M -1 s -1 .…”

“…Pulsed radiolysis of BrO 3 - solutions has been used to generate trace levels of BrO 2 in neutral and basic solutions. , This BrO 2 undergoes very rapid disproportionation with a rate proportional to [BrO 2 ] 2 and a complex dependence in [OH - ] . Micromolar concentrations of BrO 2 have been generated in sulfuric acid solution by Ce(III) reduction of BrO 3 - and by the reaction between HBrO 2 and BrO 3 - . For a brief period NaBrO 2 was available commercially, but this is no longer the case.…”

Bromine Oxides

“…Of course the BrO 2 molecule can be identified by optical absorption in the gas phase reaction of Br 2 /O 2 under flash photolysis or in the Br 2 /O 3 reactions , and it is assumed to play an intermediate part in the famous Belousov−Zhabotinsky oscillating reaction …”