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

Potassium dichromate is prepared from chromite ore(FeCr₂O₄) by the following steps:

Step1: Preparation of sodium dichromate in the reaction of Chromite ore with sodium hydroxide and oxygen gas.

4FeCr₂O₄ + 16NaOH + 7O₂ → 8Na₂CrO₄ + 2Fe₂O₃ + 8H₂O

Step2: Conversion of Sodium Chromate on reaction with concentrated Sulfuric acid gives Sodium dichromate as a product.

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

Step3: Sodium dichromate on reaction with potassium chloride converts to potassium dichromate as a product.

Na₂Cr₂O₇ + 2KCl → K₂Cr₂O₇ + 2NaCl

As potassium dichromate is less soluble than sodium chloride so, potassium dichromate is obtained in form of orange crystals.

Dichromate ion exists in equilibrium with chromate ion at around pH. However, by changing the pH they can be interconverted.