Quantitatively, ozone can be estimated with the help of potassium iodide. When ozone is made to react with potassium iodide solution buffered with a borate buffer (pH 9.2), iodine is liberated. This liberated iodine can be titrated against a standard solution of sodium thiosulphate using starch as an indicator.

The reactions involved in the process are given below.

2I^- + H₂O + O₃ → 2OH^- + I₂ + O₂

Iodide     Ozone             Iodine

I₂ + 2Na₂S₂O₃ → Na₂S₄O₆ + 2NaI

Sodium thiosulphate   Sodium tetrathionate