Write the preparation of potassium dichromate from chromite ore.

Answer: Potassium dichromate is prepared from chromite ore (FeCr₂O₄, i.e. FeO·Cr₂O₃) in three steps.

Step 1: Conversion to sodium chromate. Finely powdered ore is fused with sodium carbonate in a free supply of air:

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

The mass is extracted with water; soluble sodium chromate dissolves and insoluble Fe₂O₃ is filtered off.

Step 2: Conversion to sodium dichromate. The yellow sodium chromate solution is acidified with sulphuric acid:

2Na₂CrO₄ + H₂SO₄ → Na₂Cr₂O₇ + Na₂SO₄ + H₂O

Step 3: Conversion to potassium dichromate. The concentrated sodium dichromate solution is treated with potassium chloride:

Na₂Cr₂O₇ + 2KCl → K₂Cr₂O₇ + 2NaCl

K₂Cr₂O₇ is less soluble, so it crystallises out as orange crystals.