Potassium permanganate is prepared from pyrolusite (MnO2) ore.
The preparation involves the following steps.
1. Conversion of MnO2 to potassium manganate: Powdered ore is fused with KOH in the presence of air or oxidising agents like KNO3 . A green coloured potassium manganate is formed.
2. Oxidation of potassium manganate to potassium permanganate: Potassium manganate can be oxidised in two ways, either by chemical oxidation or electrolytic oxidation.
3. Chemical oxidation – In this method potassium manganate is treated with ozone (O3) or chlorine to get potassium permanganate.
4. Electrolytic oxidation: In this method aqueous solution of potassium manganate is electrolyzed in the presence of little alkali.
K2MnO4 ≡ 2K+ + 2MnO2-4
H2O ≡ H+ + OH-
Manganate ions are converted into permanganate ions at anode.
The purple coloured solution is concentrated by evaporation and forms crystals of potassium permanganate on cooling
