This reaction is a two part reaction where a substance is produced (iodine for example) in a slow reaction and removed in a fast reaction. When the reactants of the fast reaction runs out a change denoting the end point happens.

An example of the slower reaction is:-

This will produce the triiodide ion which can complex with starch to produce the classic blue black colour change that we use in food tests. The faster reaction will remove the triiodide before it can complex with the starch.

When the thiosulphate runs out, the triiodide ion is not being consumed and the colour change appears.


We have found success with using 0.1M solutions of the reactants Potassium Iodide, Sodium Thiosulphate (hydrated) and hydrochloric acid for the hydrogen ion. The hydrogen peroxide was used at 3%, diluted down from our stock 30%. The starch solution was made (fresh each day) into a saturated solution.