HBRO2 CE4+ REACTION AND HBRO2 DISPROPORTIONATION MEASURED IN SULFURIC-ACID-SOLUTION AT DIFFERENT ACIDITIES/

The rate of the reaction of bromous acid with Ce4+, which is responsib le for the slowing down of the autocatalytic oxidation of Ce3+ by brom ate in the Belousov-Zhabotinsky reaction, is measured directly by spec troscopic methods in sulfuric acid medium at different acidities. BrO2 radicals are identified as primary reaction products. The rate consta nt decreases from 40 000 M-1 s1 in 0.3 M H2SO4 to 1000 M-1 s-1 in 3 M H2SO4. The reaction is slowed down upon the addition of NaClO4. The ob servations are discussed on the assumption that the active cerium spec ies are sulfato complexes with one or two sulfato groups; the equilibr ium constants for the complex formation increase with increasing ionic strength leading to smaller concentrations of the active sulfato comp lexes. Additionally, the disproportionation reaction of HBrO2 was inve stigated in the same range of acidities. The rate of this reaction inc reases from 1300 M-1 s-1 in 0.3 M H2SO4 to 12 000 M-1 s-1 in 3.0 M H2S O4. We suggest that the rate-determining step is a reaction of a proto nated with an unprotonated HBrO2. At high acidities the disproportiona tion reaction is fast enough to compete with the oxidation of HBrO2 by Ce4+. These results are important for the understanding of the autoca talytic process in the Belousov-Zhabotinsky reaction, especially at hi gh sulfuric acid concentrations, and for the study of nonbromide contr olled oscillations.