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Question
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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Solution
Potassium dichromate is prepared from chromate, which in turn is obtained by the fusion of chromite ore (FeCr2O3) with sodium or potassium carbonate in free excess of air. The reaction with sodium carbonate occurs as follows:
\[\ce{4FeCr2O4 + 8Na2CO3 + 7O2 -> 8Na2CrO4 + 2Fe2O3 + 8CO2}\]
The yellow solution of sodium chromate is filtered and acidified with sulphuric acid to give a solution from which orange sodium dichromate, Na2Cr2O7.2H2O can be crystallised.
\[\ce{2Na2CrO4 + 2H+ -> Na2Cr2O7 + 2Na+ + H2O}\]
Sodium dichromate is more soluble than potassium dichromate. The latter is therefore prepared by treating the solution of sodium dichromate with potassium chloride.
\[\ce{Na2Cr2O7 + 2KCl -> K2Cr2O7 + 2NaCl}\]
Orange crystals of potassium dichromate crystallise out. The chromates and dichromates are interconvertible in an aqueous solution depending upon the pH of the solution. The oxidation state of chromium in chromate and dichromate is the same.
\[\ce{2CrO^2-_4 + 2H+ -> Cr2O^2-_7 + H2O}\]
\[\ce{Cr2O^2-_7 + 2OH- -> 2CrO^2-_4 + H2O}\]
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