ABSTRACT
The Belousov-Zhabotinsky (BZ) oscillating chemical reaction involves the oxidation of an organic compound by the bromate ion in the presence of a metal ion catalyst such as cerium(iv), manganese(ii), or ferroin. Simple periodic oscillations are generally obtained for the BZ reaction in a batch (closed) system. However, complex oscillations have been observed for the BZ reaction in batch with malonic acid and either cerium or ferroin ions as the catalyst. We report here that fascinating complex oscillations in the potential of a Pt electrode have been found in the batch BZ reaction with methylmalonic acid (MeMA) and manganese(ii). Relatively high initial concentrations of NaBrO3 and MeMA are required, and the [NaBrO3]0/[MeMA]0 ratio is the main factor determining the type of oscillations obtained. Other relevant factors are [NaBr]0, [MnSO4]0, [H2SO4]0 or [NaOH]0, temperature, and stirring rate. Complex phenomena observed include mixed mode oscillations, birhythmicity, quasiperiodicity, bursting, and possible chaos. A mechanism is proposed involving the reversible formation of a manganese(iii) complex with bromomethylmalonic acid followed by two-electron oxidation to methyltartronic acid and Mn2+.
ABSTRACT
No single-phase system has been previously reported to give significant pH oscillations in a closed (batch) reactor. We report here sustained pH oscillations in batch for the Belousov-Zhabotinsky reaction using much lower [H+]0 and much higher [BrO3-]0 than in traditional studies of this reaction. In fact, pH oscillations were obtained in the presence of only BrO3-, malonic acid (MA), and Mn2+. The amplitude, frequency, and duration of oscillations tend to depend primarily on the ratio of [BrO3-]0 to [MA]0. A critical part of the proposed mechanism involves the reversible formation of a manganese(III) complex with bromomalonic acid, followed by two-electron oxidation to tartraric acid and Mn2+. Estimates of the corresponding rate constant values for these reactions have been obtained by simulation. It is suggested that the presence of a supercatalytic reaction in H+ may be a sufficient, if not necessary, requirement for the occurrence of pH oscillations in a batch system.
