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The Iodine Clock Reaction

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.
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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The iodide ion is generated by the slow reaction between the iodate and bisulfite:
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The iodate also oxidizes the generated iodide to form iodine:
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The iodine is reduced immediately back to iodide by the bisulfite:
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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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