22.9 A Four-Color Oscillating Reaction

Chemical Concepts Demonstrated: Reversible reactions, equilibrium, complex reaction mechanisms


Solutions A and B are combined and stirred. Solution C and the ferroin indicator are then added.
  • Solution A: KBrO3 or NaBrO3 dissolved in water, H2SO4 added.
  • Solution B: Malonic acid and KBr dissolved in water.
  • Solution C: Ce(NH4)2(NO3)6 dissolved in H2SO4.
  • Ferroin Solution: 1,10-phenanthroline (C12H8N2) dissolved in water, FeSO4 added.


    Solutions A and B turn amber, then become clear after one minute.   After solution C and the ferroin solution are added, the solution cycles through green, blue, violet, and red.  The cycle will repeat these four colors for about thirty minutes.

Explanations (including important chemical equation):

    This demonstration is based on the Belousov-Zhabotinsky family of oscillating reactions.  The overall reaction involves the oxidation of malonic acid by bromate ions in dilute sulfuric acid, which is catalyzed by cerium ions.

    The mechanism for this reaction involves two different processes running simultaneously: one that produces bromine ions, and one that consumes them.   These two processes produce various intermediates, which are probably responsible for the color of the solution at any given time:

    -elemental bromine (amber)

    -ferroin complex (red when iron(II), blue when iron (III))

    -Ce4+ (yellow)