Preparation of potassium permanganate, KMnO4 from pyrolusite ore MnO2 .
(i) Pyrolusite ore is fused with KOH (or K2CO3) in presence of air (or oxidising agents such as KNO3 or KClO3) to give potassium manganate K2MnO4 (green mass).
(ii) Extraction of green mass with water followed by oxidation with chlorine (or ozone or CO2) gives potassium permanganate.
On concentrating the solution, dark purple crystals of potassium permanganate separates out.
Note: In neutral or acidic medium, manganate ion will disproportionate to permanganate ion.
Convertion of manganate to permanganate can also be carried by electrolytic oxidation.
Reactions of acidified permanganate solution with (i) iron(II) ions (ii) SO2 and (iii) oxalic acid: