Indicate the steps in the preparation of: (i) K₂Cr₂O₇ from chromite ore. (ii) KMnO₄ from pyrolusite ore.
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Potassium dichromate () is prepared from chromite ore in the following steps.
Step (1): Preparation of sodium chromate

Step (2): Conversion of sodium chromate into sodium dichromate

Step(3): Conversion of sodium dichromate to potassium dichromate

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 ion exists in equilibrium with chromate ion at pH 4. However, by changing the pH, they can be interconverted.

(ii)
Potassium permanganate () 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 KNO3 or 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

At anode, manganate ions are oxidized to permanganate ions.

Oxidation by chlorine

Oxidation by ozone

Step (1): Preparation of sodium chromate

Step (2): Conversion of sodium chromate into sodium dichromate

Step(3): Conversion of sodium dichromate to potassium dichromate

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 ion exists in equilibrium with chromate ion at pH 4. However, by changing the pH, they can be interconverted.

(ii)
Potassium permanganate () 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 KNO3 or 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

At anode, manganate ions are oxidized to permanganate ions.

Oxidation by chlorine

Oxidation by ozone

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