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

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

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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}

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

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

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

At low pH: solution is orange due to dominance of dichromate ion.

At high pH: solution is yellow due to chromate ion.


Structures of chromate and dichromate ion

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