An iodine clock reaction can be used to study the kinetics of the reaction between hydrogen peroxide and iodide ions in acidic solution.

The chemical equations for the reactions involved are:

Reaction 1:

$${\mathrm{H}}_2{\mathrm{O}}_2(\mathrm{aq})+2{\mathrm{I}}^{-}(\mathrm{aq})+2{\mathrm{H}}^{+}(\mathrm{aq})\to {\mathrm{I}}_2(\mathrm{aq})+2{\mathrm{H}}_2\mathrm{O}(\mathrm{l})$$

Reaction 2:

$${\mathrm{I}}_2(\mathrm{aq})+2{\mathrm{S}}_2{\mathrm{O}}_3^{2-}(\mathrm{aq})\to 2{\mathrm{I}}^{-}(\mathrm{aq})+{\mathrm{S}}_4{\mathrm{O}}_6^{2-}(\mathrm{aq})$$

A small, fixed volume of sodium thiosulfate solution (${\mathrm{S}}_2{\mathrm{O}}_3^{2-}$) and starch indicator are added to the reaction mixture at the start.