Question

Describe the preparation of potassium dichromate from iron chromite ore. What is the effect of increasing pH on a solution of potassium dichromate?

Answer 1

Potassium Dichromates are generally prepared from chromates which in turn are obtained by the fusion of chromite ore (FeCr₂O₄) with sodium or potassium carbonate in free excess of air

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

The yellow solution of Na₂CrO₄ is filtered and acidified with H₂SO₄ 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 prepared by treating the solution of Na₂Cr₂O₄ with KCl.

Na₂Cr₂O₇ + 2KCl → K₂Cr₂O₇ + 2NaCl (orange crystals)

(b) On increasing the PH of a solution of K₂Cr₂O₇, it behaves like a strong oxidising agent and it oxidises iodide to iodine sulphides to sulphur, tin (II) to tin (IV) and Iron (II) salts to iron (III).