Measurements and Simulations of the Acidity Dependence of the Kinetics of the Iron-Catalyzed Belousov-Zhabotinsky Reaction: Proton-Catalysis in the Electron Transfer Reaction Involving the [Fe(phen)3]3+ Species.

Abstract

The acidity dependence of the iron-catalyzed bromate-malonic acid Belousov-Zhabotinsky reaction was studied in the range 0.36 M < [H2SO4]0 < 1.20 M, and the temporal evolutions of the oscillation patterns were analyzed. The experimental results show that the period times PT i decrease exponentially with increasing acidity and that the period times parallel the decrease of the reduction times RT with increasing acidity. Simulations using the reactions of the commonly accepted core reaction mechanism failed to match the measurements even in a qualitative fashion. However, we found that compelling agreement between the experiments and the simulations can be achieved over the entire range with the inclusion of second-order proton-catalysis of the oxidation of bromomalonic acid (BrMA) by the [Fe(phen)3]3+ species in the reaction identified in this paper as reaction 9 (R9), and this [H+] dependence is informative about the species involved in the outer sphere electron transfer reaction. The trication [Fe(phen)3]3+ species is stabilized by ion pairing and solvation, and one may anticipate the presence of [Fe(phen)3(HSO4) n(H2O) m](3- n)+ species ( n = 0-3). Our results suggest that the removal of aggregating HSO4- ions by protonation creates a better oxidant and facilitates the approach of the reductant BrMA, and the second-order [H+] dependence further suggests that BrMA is primarily oxidized by a doubly charged [Fe(phen)3(HSO4)1(L) k]2+ species. Considering the complexity of the BZ system and the uncertainties in the many reaction rate constants, we were somewhat surprised to find this high level of agreement by (just) the replacement of R9 by R9'. In fact, the near-quantitative agreement presents a powerful corroboration of the core reaction mechanism of the BrMA-rich BZ reaction, and the replacement of R9 by R9' extends the validity of this core reaction mechanism to acidities above and below the typical acidity of BZ reactions ([H+] ≈ 1 M).