Laboratory Preparation of Potassium Dichromate

Preparation of potassium dichromate from chrome iron ore.

The preparation of Potassium dichromate (K2Cr2O7) from its ore involves the following steps:


(i) Conversion of chrome iron ore to sodium chromate

The powdered ore is mixed with Na2CO3 and quick lime, then roasted in a reverberatory furnace with free expose to air.

4FeO.Cr2O3 + 8Na2CO3 + 7O2 → 8Na2CrO4 + 2Fe2O3 + 8CO2

(ii) Conversion of Na2CrO4 to Na2Cr2O7

Sodium chromate solution so obtained is filtered and treated with con.H2SO4, when sodium chromate is converted to sodium dichromate.

2Na2CrO4 + H2SO4 → Na2Cr2O7 + Na2SO4 + H2O

(iii) Conversion of sodium dichromate into potassium dichromate

Hot concentrated solution of sodium dichromate is treated with KCl, when potassium dichromate, being much less soluble than sodium salt, crystallizes out on cooling.

Na2Cr2O7 + 2KCl → K2Cr2O7 + 2NaCl

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