What Everyone Should Know About the Belousov-Zhabotinsky Reaction

In the early 1950s a Soviet biochemist, Boris P. Belousov, was trying to develop a simple chemical model of the oxidation of organic molecules in living cells. Central to these pathways is the Krebs cycle, whereby organic acids are oxidized to CO2 and H2O. In aerobic organisms, oxygen is the oxidizing agent, and the reactions are catalyzed by enzymes and electron-transport proteins, many of which rely on iron ions (Fe2+/Fe3+) to move electrons around. In his testtube version of metabolism, Belousov used citric acid (one of the intermediates of the Krebs cycle) as an organic substrate, bromate ions (BrO3-) as oxidizing agent, and cerium ions as catalyst. Any chemist would expect the reaction to proceed monotonically to equilibrium, perhaps showing one visible sign of progress by changing from a colorless solution (cerium in the reduced state, Ce3+) to pale yellow (the oxidized state, Ce4+). So we can imagine Belousov’s surprise when his reaction mixture turned yellow then colorless, then yellow again and colorless, oscillating dozens of times between oxidized and reduced states (Fig. 1).