Kinetics of Disproportionation and pKa of Bromous Acid

The bromous acid dissociation constant is essentially identical to the value previously obtained from a kinetics study of the bromine reaction.

 

The bromous acid dissociation constant is essentially identical to the value previously obtained from a kinetics study of the bromine reaction.

Introduction
The first relatively unambiguous determination of the pK, of bromous acid was achieved by studying the kinetics of the reaction between bromine and iodide ion.2 Because thisvalue differed significantly from some earlier estimates, we have sought other reactions of bromine that could be used for the same purpose. The disproportionation of bromine, is such a reaction. Its kinetics can provide a second measurement of the pK, of HBr02, which affords comparison of this value with the previously determined value.
We report here measurements of the rate law for the kinetics of disproportionation of bromine at pH 3.5-8.0 and suggest a mechanism for this reaction. Qualitatively, the results agree with earlier studies of bromine decomposition at pH 6.2- 8.5, except that we find no [Br]- dependence. From our results at pH 3.5-5.6, we present a new measurement of the pK, of HBr02 that confirms our previously reported value.

Disproportionation Kinetics of Bromous Acid
The bromous acid pK, of 3.43 f 0.9 X 10-4 M that emerges from this kinetics study of bromine disproportionation is essentially identical with the value measured frop the kinetics of the reaction between bromine and iodide at the same ionic strength (0.06 M).2 Both methods have drawbacks, however. For example, the bromine reaction becomes so rapid at low pH that it encroaches on the stopped-flow resolution time, which may lead to errors in measurements of the initial velocities. It is easier to follow the disproportionation of bromine, because it is slower. However, products formed by the decomposition of bromine, e.g., HOBr, Brz, and Br3-, absorb UV light in the same region used to follow this reaction. Our inability to subtract these additional absorbances from our experimental data introduces some error into our results. These errors cannot be very large, because we are measuring initial velocities, and at the start of the reaction the concentrations (and, of course, the absorbances) of the products must be small.

Phosphate Buffer
Phosphate buffer kinetics studies suffer from numerous experimental problems described above, such as an unaccountable induction period. Furthermore, an interaction between phosphate and some form of bromine was noted while studying bromine iodide reactions. Because of these problems, we do not rely on the rate and equilibrium constants determined from kinetics studies with this buffer. (There is no indication of general acid catalysis, because a change in buffer concentration does not change reaction rate, provided [H+] is unchagned.) Two kinetics studies of the bromous acid disproportionation reaction in sulfuric acid media yield apparent rate constants in the range. These values are significantly higher than our value of k2 and suggest a change in reaction pathway due, perhaps, to formation of H2BrOz+ and its reaction with HBr02.


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