Mechanism for the Belusov-Zhabotinsky Reaction

In this rendetion of the reaction the color change is due to the ferrous phenothroline to ferric phenthroline complexes. Phenanthroline is a nitrogen containing ring compound that forms colored complexes with iron.

As the ferrous complex, which is red, is oxidized to the ferric complex, a bright blue, the dramatic color change occurs. This redox step is therefore monitored by the color change of the complex. There are two such steps in the mechanism. Furthermore, the molecule malonic acid takes part in the oxidation-reduction step after being brominated. There is loss in this step and varying production of bromide ion. This complexity leads to irregularity in the oscillations.

The Mechanism

1. The overall stoichiometry in step A is
   BrO_3^- + 5Br^- + 6H^+ ---> 3Br_2 + 3H_2O
   which removes bromide ion. As bromide ion decreases each of the reaction rates of the three steps decreases.
   
2. As HBrO_2 builds up it competes with Br^- in reacting with bromate, BrO_3^-.
   
3. In step B the HBrO_2 +BrO_3^- reaction forms BrO_2 which oxidizes the ferrous ion in the complex turning it from read to blue.
   
4. Step C produces bromide ion and reduces the catalyst, the ferric bound in the complex. One of the complicating features is that bromine is attached to malonic acid.

Scott, Stephen K., Chemical chaos, Oxford University Press, (1993). Chapter 8.

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