Potassium Permanganate, (KMnO₄)

Potassium permanganate is a salt of an unstable acid HMnO₄ (permanganic acid). The Mn is an +7 state in this compound.

Preparation : Potassium permanganate is obtained from pyrolusite as follows.

Conversion of pyrolusite to potassium manganate : When manganese dioxide is fused with potassium hydroxide in the presence of air or an oxidising agent such as potassium nitrate or chlorate, potassium manganate is formed, possibly via potassium manganite.

By chemical method : The fused dark-green mass is extracted with a small quantity of water. The filtrate is warmed and treated with a current of ozone, chlorine or carbon dioxide. Potassium manganate gets oxidized to potassium permanganate and the hydrated manganese dioxide precipitates out. The reactions taking place are,

When CO₂ is passed

2CO₂ + 4KOH → 2K₂CO₃ + 2H₂O

When chlorine or ozone is passed

2K₂MnO₄ + Cl₂ → 2KMnO₄ + 2KCl

2K₂MnO₄ + O₃ + H₂O → 2KMnO₄ + 2KOH + O₂(g)
The purple solution so obtained is concentrated and dark purple, needle-like crystals having metallic lustre are obtained.

Electrolytic method : Presently, potassium manganate (K₂MnO₄) is oxidized electrolytically. The electrode reactions are,

At anode: 2MnO₄^2– → 2MnO₄^– + 2e^–

At cathode: 2H⁺ + 2e^– → H₂(g)

The purple solution containing KMnO₄ is evaporated under controlled condition to get crystalline sample of potassium permanganate.

Physical properties

KMnO₄ crystallizes as dark purple crystals with greenish luster (m.p. 523 K).

It is soluble in water to an extent of 6.5g per 100g at room temperature. The aqueous solution of KMnO₄ has a purple colour.