22.8 The Blue/Amber/Colorless Oscillating Reaction

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


Three solutions are combined in alphabetical order.
  • Solution A: 30% H2O2 diluted in water.
  • Solution B: KIO3 dissolved in water, H2SO4 added, and diluted.
  • Solution C: Malonic acid and MnSO4 * H2O dissolved in water, starch solution added, and diluted.


    The resulting solution turns fron amber to blue-black.  The blue slowly fades to colorless, and the reaction begins again.  During the course of the reaction, bubbles are evolved.

Explanations (including important chemical equations):

    Although the complete mechanism is uncertain, the reaction is believed to be an indirect route to decomposing H2O2 into H2O and O2, related to the H2O2/IO3- oscillating reation.  If so, the amber color results from the production of I2 in the absence of I -, while the blue color results from the simultaneous presence of both I2 and I - leading to the blue starch-I3- complex.  The bubbles are oxygen gas.  The following steps have been proposed:

    5 H2O2 (aq) + I2 (aq) --> 2 HIO3 (aq) + 4 H2O (l)

    5 H2O2 (aq) + 2 HIO3 (aq) --> 5 O2 (g) + I2 (aq) + 6 H2O (l)

    The functions of the malonic acid and the manganese ion may be to increase the size and frequency of the I2 and I - fluctuations.