Indicate the steps in the preparation of $K_{2} {C r}_{2} O_{7}$ from chromite ore.

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Solution

Potassium dichromate: ( $K_{2} {C r}_{2} O_{7}$ ) is prepared from chromite ore $(F e {C l}_{2} O_{4})$ in the following steps:

(1) Preparation of sodium chromate

$4 F e {C r}_{2} O_{4} + 16 N a O H + 7 O_{2} \to 8 {N a}_{2} C r O_{4} + 2 {F e}_{2} O_{3} + 8 H_{2} O$

(2) Conversion of sodium chromate into sodium dichromate

$2 {N a}_{2} C r O_{4} + c o n c . H_{2} {S O}_{4} \to {N a}_{2} {C r}_{2} O_{7} + {N a}_{2} {S O}_{4} + H_{2} O$

(3) Conversion of sodium dichromate to potassium dichromate

${N a}_{2} {C r}_{2} O_{7} + 2 K C l \to 2 N a C l + K_{2} {C r}_{2} O_{7}$

Potassium chloride being less soluble than sodium chloride is obtained in the form of orange coloured crystals and can be removed by filteration.

The dichromate ion ( ${C r}_{2} O_{7}^{2 -}$ ) exists in equilibrium with chromate $(C r O_{4}^{2 -})$ ion at pH. $4$ . However, by changing the pH they can be interconverted.

$2 C r O_{4}^{2 -} c h r o m a t e (y e l l o w) a c i d ⇋ a l k a l i 2 H C r O_{4}^{-} H y d r o g e n c h r o m a t e a c i d ⇋ a l k a l i {C r}_{2} O_{7}^{2 -} D i c h r o m a t e (o r a n g e)$

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