Question

Indicate the steps in the preparation of K₂Cr₂O₇ from chromite ore.

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

Dichromates are generally prepared from chromate, which in turn is obtained by the fusion of chromite ore (FeCr₂O₄) with sodium or potassium carbonate in free air. The reaction with sodium carbonate occurs as follows:

\[\ce{4FeCr_2O_4 + 8Na2CO3 + 7O_2 ->8Na_2CrO_4 + 2Fe_2O_3 + 8CO2}\]

The yellow solution of sodium chromate is filtered and acidified with sulphuric acid to give a solution from which orange sodium dichromate, Na₂Cr₂O₇. 2H₂O can be crystallised.

\[\ce{2Na_2CrO_4 + 2H^+ -> Na_2Cr_2O_7 + 2Na^+ + H_2O}\]

Sodium dichromate is more soluble than potassium dichromate. Therefore, the latter is prepared by treating a sodium dichromate solution with potassium chloride.

\[\ce{Na_2Cr_2O_7 + 2KCl -> K_2Cr_2O_7 + 2NaCl}\]

Orange crystals of potassium dichromate crystallise out. The chromates and dichromates are interconvertible in an aqueous solution, depending on the pH of the solution. The oxidation state of chromium in chromate and dichromate is the same.

\[\ce{2CrO^{2-}_4 + 2H^+ -> Cr2O^{2-}_7 + H2O}\]

\[\ce{Cr2O^{2-}_7 + 2OH^- -> 2CrO^2-_4 + H2O}\]

Hence, when the pH is increased, making the solution alkaline, dichromate ions (orange in colour) are converted into chromate ions, and the solution becomes yellow.