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The Iodine Clock Reaction

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The iodine clock reaction (also known as the Harcourt–Esson reaction) is a classic chemical clock experiment for displaying chemical kinetics in action; it was discovered by Hans Heinrich Landolt in 1886. Two colorless solutions are mixed and at first there is no visible reaction. After a short, predictable time delay, the liquid suddenly turns a shade of dark blue.

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Adjustment of the relative amounts of the iodate and the bisulfite determines the time interval (shown on the graph) before the blue color appears.

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Contributed by: Benson R. Sundheim (December 2011)
Open content licensed under CC BY-NC-SA

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The iodide ion is generated by the slow reaction between the iodate and bisulfite: .

The iodate also oxidizes the generated iodide to form iodine: .

The iodine is reduced immediately back to iodide by the bisulfite: .

When the bisulfite is fully consumed, the iodine will accumulate (i.e., not be reduced by the bisulfite) and form the dark blue complex with starch.

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