Iodine Clock Reaction - Iodate Variation

Iodate Variation

An alternative protocol uses a solution of iodate ion (for instance potassium iodate) to which an acidified solution (again with sulfuric acid) of sodium bisulfite is added.

In this protocol, iodide ion is generated by the following slow reaction between the iodate and bisulfite:

IO₃− + 3 HSO₃− → I− + 3 HSO₄−

This is the rate determining step. The iodate in excess will oxidize the iodide generated above to form iodine:

IO₃− + 5 I− + 6 H+ → 3 I₂ + 3 H₂O

However, the iodine is reduced immediately back to iodide by the bisulfite:

I₂ + HSO₃− + H₂O → 2 I− + HSO₄− + 2 H+

When the bisulfite is fully consumed, the iodine will survive (i.e., no reduction by the bisulfite) to form the dark blue complex with starch.