Video: The starch-iodine clock reaction

This particular version is, in fact, the Landolt Reaction. The first, very slow step, is the rate determining step in which iodate is reduced to iodide

HSO3-(aq) + H2O(l) ® HSO4-(aq) + 2 H+(aq) + 2 e-
IO3-(aq) + 6 H+(aq) ® I-(aq) + 3 H2O(l) (l)

The iodide ion then undergoes a fast conproportionation reaction with excess iodate ion:

2 I-(aq) ® I2(aq) + 2 e-
2 IO3-(aq) + 12 H+(aq) + 10 e- ® I2>(aq) + 6 H2O(l)

The iodine is then reduced to iodide ion again in an ‘immeasurably fast’ reaction with hydrogen sulfite ion:

I2(aq) + 2 e- ® 2 I-(aq)
HSO3-(aq) + H2O(l) ® HSO4-(aq) + 2 H+(aq) + 2 e-

It is only when all the hydrogen sulfite ion is completely consumed that the iodine concentration suddenly starts to build up and the characteristic deep blue-black starch-iodine complex forms (this consists of the starch molecules wrapping themselves helically around a chain of iodine molecules).