Redox Titration

Titration of Ferrous Ammonium Sulphate solution against Potassium Permanganate solution

Potassium permanganate is an oxidising agent, both in alkaline and acidic medium. Oxidation is always accompanied by reduction. In this titration, the ferrous ion is oxidised to ferric ion by permanganate ion, [Mn (VII)] and at the same time permanganate ion is reduced to manganese (II) [Mn (II)].

The following reactions take place

MnO₄^– + 8 H⁺ + 5 e^– → Mn²⁺ + 4 H₂O

Fe²⁺ → Fe³⁺ + e^–

The overall reaction is

According to the stoichiometric relation, one mole of the oxidising agent, potassium permanganate, reduces 5 moles of reducing agent, ferrous ammonium sulphate.

Potassium permanganate is a self indicator, a pink colour appears and persists at the end point. To calculate the strength, the following relation is used:

a₂ M₁ V₁ = a₁ M₂ V₂

Experiment: To find out the Molarity and strength of given potassium permanganate solution by titrating against M/50 Mohr’s salt (Ferrous ammonium sulphate) solution.

How To Perform Experiment

Apparatus Required

Burette, pipette, clamp stand, white tile or paper, conical flask, wash bottle, beaker

Chemicals Required

M/50 Mohr salt solution, dil (4N) H₂SO₄, KMnO₄ solution

Indicator

KMnO₄ acts as self indicator

End Point

Colourless to permanent pink

Procedure

Wash and rinse the burette and pipette with distilled water and then with the solution to be filled in them. Fill the burette with the given KMnO₄ solution and pipette out 20 ml Mohr’s salt solution (M/50) in a washed titration or conical flask.

Add one test tube (20 ml) full of dilute sulphuric acid (4 N) to the Mohr’s salt solution taken in the conical flask. Note the intial reading of burette and start adding KMnO₄ solution from the burette till a permanent pink colour is imparted to the solution in the conical flask.

Note the final reading. Repeat the process to get three concordant reading.