Question

Explain the preparation of K2Cr2O7 from chromite ore and write the chemical equations involved?

Answer 1

Potassium dichromate is prepared from chromite ore (FeCr₂O₄) in the following steps.

Step (1):

Preparation of sodium chromate

4FeCr₂O₄ + 16NaOH + 7O₂ → 8NaCrO₄ + 2Fe₂O₃ + 8H₂O

Step (2):

Conversion of sodium chromate into sodium dichromate

2Na₂CrO₄ + conc.H₂SO₄ → Na₂Cr₂O₇ + Na₂SO₄ + H₂O

Step(3): Conversion of sodium dichromate to potassium dichromate

Na₂Cr₂O₇ + 2KCl → K₂Cr₂O₇ + 2NaCl

Potassium dichromate being less soluble than sodium chloride is obtained in the form of orange coloured crystals and can be removed by filtration.

The dichromate (Cr₂O^2-₇) ion exists in equilibrium with chromate (CrO^2-₄) ion at pH 4. However, by changing the pH, they can be interconverted.