1) Preparation of K₂Cr₂O₇

Dichromates are generally prepared from chromate, which in turn are obtained by the fusion of chromite ore (FeCr₂O₄ ) with sodium or potassium carbonate in free access of air. The reaction with sodium carbonate occurs as follows:

4 FeCr₂O₄ + 8 Na₂CO₃ + 7 O₂ → 8 Na₂CrO₄ + 2 Fe₂O₃ + 8 CO₂

The yellow solution of sodium chromate is filtered and acidified with sulphuric acid to give a solution from which orange sodium dichromate, Na₂Cr₂O₇.2H₂O can be crystallised.

2Na₂CrO₄ + 2H⁺ → Na₂Cr₂O₇ + 2Na⁺ + H₂O

Sodium dichromate is more soluble than potassium dichromate. The latter is therefore, prepared by treating the solution of sodium dichromate with potassium chloride.

Na₂Cr₂O₇ + 2KCl → K₂Cr₂O₇ + 2NaCl

Orange crystals of potassium dichromate crystallise out.

2) Oxidising action of K₂Cr₂O₇

In acidic solution, it acts as oxidising agent.

For e.g. It oxidises Fe²⁺ to Fe³⁺ 

Cr₂O₇^2– + 14 H⁺ + 6 Fe²⁺ → 2 Cr³⁺ + 6 Fe³⁺ + 7 H₂O