When three colorless solutions are stirred together, the result shortly becomes amber, and then quite suddenly blue. The colorless, amber, and blue sequence of colors is repeated in a cycle of several seconds for some minutes. This is a good "gee whiz" demo. It can also be shown in Chem 480AB as an example of an oscillating reaction.

3 1-L bottles containing reactant solutions (below) from preproom
solution 1: hydrogen peroxide, ca. 12%
solution 2: potassium iodate in aq. sulfuric acid
solution 3: aq. Malonic acid, manganese(II) sulfate, and starch
one beaker, 500 mL to 2L in size
magnetic stirrer
one 2-in stirring bar

Place the beaker to be used for the presentation with the stir bar on the magnetic stirrer. Fill it one-quarter full with solution 1, add an equal volume of solution 2, and start the stirrer. Add an equal volume of solution 3. Color oscillations will begin in about half a minute.

This reaction, called the Briggs-Rauscher Reaction, does not really oscillate in the sense that the reactants are restored to their initial state. The net reaction, which is simply

2H2O2 2H2O + O2

Although this explains part of what is going on, it does not make clear why I2 is present during some stages of the reaction (giving it a blue color because of the complex I2 forms with starch) and absent during others. An extensive discussion of this reaction and of oscillating reactions in general is given in the Reference.

Iodine is the ultimate product left in the reaction mixture. It may be disposed of by adding sodium thiosulfate to the mixture and stirring. Care should be exercised because the reaction between iodine and thiosulfate is very exothermic. Iodine also stains the skin and white cotton shirts. Do not drink the solutions. Do not drop the stirrer on your foot.

B. Z. Shaksashiri, Chemical Demonstrations, Vol. 2, p. 249.

T. S. Briggs and W. C. Rauscher, J. Chem. Educ., 50, 496 (1973).

Oscillating reactions are very striking demonstrations, and others are given in the Reference, as well as alternative presentations of the Briggs-Rauscher reaction. The Reference also gives a recipe for the Briggs-Rauscher reaction based on 3% H2O2, although one that gives slower changes.