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Question 26

Indicate the steps in the preparation of:

(i) K2Cr2O7 from chromite ore.

(ii) KMnO4 from pyrolusite ore.


(i) Potassium dichromate (K2Cr2O7) is prepared from chromite ore (FeCr2O4) in the following steps.

Potassium chloride being less soluble than sodium chloride is obtained in the form of orange coloured crystals and can be removed by filtration. The dichromate (Cr2O2-7) ion exists in equilibrium with chromate (CrO2-4) ion at pH 4. However, by changing the pH, they can be interconverted.

(ii) Potassium permanganate (KMnO4) can be prepared from pyrolusite (MnO2). The ore is fused with KOH in the presence of either atmospheric oxygen or an oxidising agent, such as KNO3or KClO4, to give K2MnO4.

The green mass can be extracted with water and then oxidized either electrolytically or by passing chlorine/ozone into the solution.

Electrolytic oxidation


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