Iodine Clock Reaction - Persulfate Variation

Persulfate Variation

This clock reaction uses sodium, potassium or ammonium persulfate to oxidize iodide ions to iodine. Sodium thiosulfate is used to reduce iodine back to iodide before the iodine can complex with the starch to form the characteristic blue-black colour.

Iodine is generated:

2 I− + S₂O₈2− → I₂ + 2 SO₄2−

And is then removed:

I₂ + 2 S₂O₃2− → 2 I− + S₄O₆2−

Once all the thiosulfate is consumed the iodine may form a complex with the starch. Potassium persulfate is less soluble (cfr. Salters website) while ammonium persulfate has a higher solubility and is used instead in the reaction described in examples from Oxford University.