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

Preparation of potassium dichromate $K_{2}C{r}_2{O}_7$ from chromite ore $FeO\cdot C{r}_2{O}_3\left(FeC{r}_2{O}_4\right)$:
(i) Chromite ore is fused with $NaOH$ or sodium carbonate in air to form sodium chromate.
$4FeC{r}_2{O}_4+16NaOH+7O_2\to 8N{a}_{2}Cr{O}_4+2F{e}_2{O}_3+8H_{2}O$

(ii) Sodium chromate solution is filtered and acidified to form sodium dichromate
$2N{a}_{2}Cr{O}_4+H_{2}S{O}_4\to N{a}_{2}C{r}_2{O}_7+N{a}_{2}S{O}_4+H_{2}O$

The solution is cooled to remove crystallized sodium sulphate. Sodium dichromate remains soluble.

(iii) Equimolar proportions of a hot solution of sodium dichromate and KCl are mixed to form potassium dichromate.
$N{a}_{2}C{r}_2{O}_7+2KCl\to K_{2}C{r}_2{O}_7+2NaCl$

Sodium chloride precipitates out in hot solution and is removed by filtration. On cooling, potassium dichromate separates out from the mother liquor.
At pH 4, there is an equilibrium between chromate ions and dichromate ions

$\underset{\text{Dichromate ion (orange red)}}{C{r}_2{O}_7^{2-}}+H_{2}O\rightleftharpoons \underset{\text{Chromate ion (yellow)}}{2Cr{O}_4^{2-}}+2H^{+}$

By adding alkali, pH can be increased and the equilibrium reaction will shift in the forward direction to produce more chromate ions (yellow color). This is in accordance with Le chatelier's principle. By adding an acid, pH can be decreased and the equilibrium reaction will shift in the backward direction to produce more dichromate ions (orange-red color).