Complex oscillations in the Belousov-Zhabotinsky batch reaction with methylmalonic acid and manganese(ii)

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⁺.