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

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Solution

(i) Preparation of $K_{2} C r_{2} O_{7}$ from chromite ore
(a) Potassium dichromate is obtained by the fusion of chromite ore $(F e C r_{2} O_{4})$ with sodium (or potassium) carbonate in the presence of excess air.
$4 F e C r_{2} O_{4} + 8 N a_{2} C O_{3} + 7 O_{2} \to 8 N a_{2} C r O_{4} (\begin{matrix} Y e l l o w s o l u t i o n \end{matrix}) + 2 F e_{2} O_{3} + 8 C O_{2}$

(b) The solution is filtered and treated with sulphuric acid.
$2 N a_{2} C r O_{4} + 2 H^{+} (F r o m H_{2} S O_{4}) \to N a_{2} C r_{2} O_{7} \begin{matrix} S o d i u m d i c h r o m a t e (o r a n g e) \end{matrix} (s) + 2 N a^{+} + H_{2} O$

(c) Now sodium dichromate is treated with potassium chloride.
As a result, potassium dichromate is produced.
$N a_{2} C r_{2} O_{7} + 2 K C l \to K_{2} C r_{2} O_{7} \begin{matrix} P o t a s s i u m d i c h r o m a t e c r y s t a l s \end{matrix} + 2 N a C l$

(ii) Effect of increasing pH on $K_{2} C r_{2} O_{7}$ solution
$C r_{2} O_{7}^{2 -} O r a n g e + 2 O H^{-} \to 2 C r O_{4}^{2 -} Y e l l o w + H_{2} O$
On increasing pH; $K_{2} C r_{2} O_{7}$ changes into $K_{2} C r O_{4}$ (orange to yellow).

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