Question

Mention the different steps for the preparation of potassium dichromate from iron chromite ore and draw the structure of dichromate ion.

Hint:

Fuse chromite with Na₂CO₃ in air to chromate, acidify to dichromate, then add KCl. The Cr₂O₇^2- ion is two CrO₄ tetrahedra sharing one oxygen.

Answer 1

Potassium dichromate (K₂Cr₂O₇) is prepared from chromite ore, FeCr₂O₄ (iron chromite), in three main steps.
Step 1 (Conversion of chromite to sodium chromate): The powdered ore is fused with molten sodium carbonate (or NaOH) in excess air. Chromium is oxidised to the +6 state, giving yellow sodium chromate:
4FeCr₂O₄ + 8Na₂CO₃ + 7O₂ → 8Na₂CrO₄ + 2Fe₂O₃ + 8CO₂
Step 2 (Sodium chromate to sodium dichromate): The chromate solution is acidified with concentrated sulphuric acid, which converts orange-yellow chromate into orange sodium dichromate:
2Na₂CrO₄ + H₂SO₄ → Na₂Cr₂O₇ + Na₂SO₄ + H₂O
Step 3 (Sodium dichromate to potassium dichromate): The hot sodium dichromate solution is treated with potassium chloride. Potassium dichromate, being less soluble, crystallises out on cooling:
Na₂Cr₂O₇ + 2KCl → K₂Cr₂O₇ + 2NaCl
Structure of dichromate ion (Cr₂O₇^2-): It consists of two CrO₄ tetrahedra joined by sharing one corner oxygen atom, giving a Cr-O-Cr bridge. Each chromium is in the +6 oxidation state, tetrahedrally surrounded by oxygen; the bridging Cr-O-Cr angle is about 126 degrees.