Introduction

This is a redox titration. In this reaction potassium dichromate acts as an oxidizing agent in acidic medium. It oxidizes FAS (Fe²⁺) in Ferric sulphate (Fe³⁺). As the K₂Cr₂O₇ is added in FAS, inintially green color appears due to the formation of the reduction product Cr₂(SO₄)₃ , (Cr³⁺) which on further addition of K₂Cr₂O₇ changes to a purple color.

The reactions involved are as follows:

K₂Cr₂O₇ + 4H₂SO₄ → K₂SO₄ + Cr₂(SO₄)₃ + 4H₂O + 3[O]

6FeSO₄ + 3H₂SO₄ + 3[O] → 3Fe₂(SO₂)₄ + 3H₂O

The complete reaction is:

6[FeSO₄ (NH₄)₂SO₄·6H₂O] + 7H₂SO₄ + K₂Cr₂O₇ → 3Fe₂(SO₄)₃ + 6(NH₄)₂SO₄ + Cr₂(SO₄) + K₂SO₄ + 4H₂O