Arrange the following steps in the preparation of potassium dichromate from chromite ore in the correct sequence.
(A) The solution of sodium dichromate is treated with potassium chloride.
(B) Chromite ore is fused with sodium carbonate in free access of air.
(C) The yellow solution obtained is acidified with sulphuric acid.
(D) Orange crystals of potassium dichromate crystallise out.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (B), (C), (A), (D)
(B) Chromite ore is fused with sodium carbonate in free access of air. — First. Air oxidises Cr(III) of the ore to Cr(VI), giving sodium chromate, which dissolves to form a yellow solution:
\( \displaystyle \ce{4FeCr2O4 + 8Na2CO3 + 7O2 -> 8Na2CrO4 + 2Fe2O3 + 8CO2} \)
(C) The yellow solution obtained is acidified with sulphuric acid. — Second. In acid, chromate changes into orange sodium dichromate:
\( \displaystyle \ce{2Na2CrO4 + 2H+ -> Na2Cr2O7 + 2Na+ + H2O} \)
(A) The solution of sodium dichromate is treated with potassium chloride. — Third. Sodium dichromate is more soluble than potassium dichromate, so adding KCl gives the less soluble potassium salt:
\( \displaystyle \ce{Na2Cr2O7 + 2KCl -> K2Cr2O7 + 2NaCl} \)
(D) Orange crystals of potassium dichromate crystallise out. — Fourth. The potassium dichromate formed in step (A) separates as orange crystals; it cannot crystallise before KCl has been added.
NCERT (2026-27 edition):
4.4.1 Oxides and Oxoanions of Metals – “…Dichromates are generally prepared from chromate, which in turn are obtained by the fusion of chromite ore (FeCr2O4) with sodium or potassium carbonate in free access of air. … 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.”
“Sodium dichromate is more soluble than potassium dichromate. The latter is therefore, prepared by treating the solution of sodium dichromate with potassium chloride.” … “Orange crystals of potassium dichromate crystallise out.”