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Indicate the steps in the preparation of : K_{2}Cr_{2}O_{7} from chromite ore.

8.26 Indicate the steps in the preparation of:

(i) K_{2}Cr_{2}O_{7}  from chromite ore.

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(i) Potassium dichromate is obtained from the fusion of chromite ore (FeCr_{2}O_{4}) with sodium and potassium carbonate in the free supply of air.

4FeCr_{2}O_{4}+8Na_{2}CO_{3}+7O_{2}\rightarrow 8Na_{2}CrO_{4}+2Fe_{2}O_{3}+8CO_{2}

(ii) Sodium chromate is filtered and acidified with sulphuric acid (H_{2}SO_{4}) to form sodium dichromate, (Na_{2}Cr_{2}O_{7}.2H_{2}O) can be crystallised

2Na_{2}CrO_{4}+2H^{+}\rightarrow Na_{2}Cr_{2}O_{7}+2Na^{+}+H_{2}O

(iii) Sodium dichromate is more soluble than potassium dichromate. So, treat the solution of dichromate with the potassium chloride(KCl)

Na_{2}Cr_{2}O_{7}+KCl\rightarrow K_{2}Cr_{2}O_{7}+2NaCl

The chromate and dichromate are interconvertible in aqueous solution at pH 4


Structures of chromate and dichromate ion

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