On the other hand, it is possible to estimate active oxidizing agents like permanganates, dichromate, hydrogen peroxide and cupric ions by treating with excess of potassium iodide to liberate iodine. The liberated iodine is then estimated by titration against standard sodium thiosulphate solution. The following reactions are involved in these titrations:

With acidified KMnO₄ solution
                                2KMnO₄ + 3H₂SO₄    →    K₂SO₄ + 2MnSO₄ + 3H₂O + 5(O)
                                     2KI + O + H₂SO₄    →    K₂SO₄ + H₂O + I₂

With acidified K₂Cr₂O₇ solution
                                K₂Cr₂O₇ + 4H₂SO₄    →    K₂SO₄ + Cr₂(SO₄)₃ + 4H₂O + 3(O)
                                      2KI + O + H₂SO₄   →   K₂SO₄ + H₂O + I₂

Withe copper sulphate solution
                                        CuSO₄ + 2KI     →   CuI₂ + K₂SO₄
                                                       2CuI₂   →   Cu₂I₂ + I₂ .

All those titrations in which an oxidizing agent is estimated by first reacting with potassium iodine and estimating the liberated iodine with standard sodium thiosulphate solution are known as Iodomertric titrations.

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