Potassium Permanganate

Potassium Permanganate

Aug 26, 2022, 16:45 IST

Introduction of Potassium Permanganate, (KMnO ₄ )

This is the most important and well known salt of permanganic acid. It is prepared from the pyrolusite ore.

Preparation of Potassium Permanganate

1. Disproportionation of Potassium Manganate (K ₂ MnO ₄ ):  Manganate ion disproportionate into manganate ion and manganese dioxide in acidic medium.
2. Oxidation of K ₂ MnO ₄ by Cr ₂ or O ₃
3. Commercial production:

Alkaline oxidative fusion of pyrolusite (MnO ₂ ) followed by the electrolytic oxidation of the manganate ion so formed.The pyrolucite is first fusel with KOH in presence of some oxidizing agent (KNO ₃ or KClO ₃ ) or atmospheric oxygen in a muffle furnace to give K ₂ NMnO ₄ as green mass.  The resulting mass is reached with water and the solution is oxidized to KMnO4 either by Cl ₂ , O ₃ or CO ₂ as above or by electrolytic oxidation.

Properties of Potassium Permanganate

It is purple coloured crystalline compound. It is fairly soluble in water. When heated alone or with an alkali, it decomposes evolving oxygen. On treatment with conc. H ₂ SO ₄ , it forms manganese heptoxide via permanganyl sulphate which decomposes explosively on heating.Potassium permanganate is a powerful oxidising agent. A mixture of sulphur, charcoal and KMnO ₄ forms an explosive powder. A mixture of oxalic acid and KMnO ₄ catches fire spontaneous after a few seconds. The same thing happens when glycerine is poured over powdered KMnO ₄ .

Potassium permanganate acts as an oxidising agent in alkaline, neutral or acidic solutions.

(a) In alkaline solution: KMnO ₄ is first reduces to manganate then to insoluble manganese dioxide. Colour changes first from purple to green and finally becomes colourless. However, brownish precipitate is formed.

The important oxidation reactions of Potassium Permanganate are:

In Acidic Medium:

(i) Ferrous salts are oxidised to ferric salts.

(ii) Potassium iodide is converted to iodine.

(iii) H ₂ S is oxidised to sulphur.

(iv) SO ₂ is oxidised to H ₂ SO ₄ .

(v) Nitrites are oxidised to nitrates.

(vi) Oxalic acid is oxidised to CO ₂ .

(vii) Hydrogen halides (HCI, HBr or HI) are oxidized into X ₂ (halogen).

In Neutral Medium:

(i) H ₂ S is oxidised to sulphur.

(ii) Manganese sulphate is oxidised to MnO ₂

(iii) Sodium thiosuiphate is oxidised to sulphate and sulphur.

In Alkaline Medium

(i) Iodide is oxidized to iodate.

(ii) Ethylene is oxidized to ethylene glycol.

In alkaline medium it is called Baeyer’s reagent.

Uses of Potassium Permanganate

i) Oxidizing agent in laboratory & industry.

ii) in volumetric analysis  as intermediate solution generally.

iii) As Baner’s reagent in organic chemistry for test of unsaturation the products being colourless, the decolourisation of pink colour of the reagent takes place.

iv) In qualitative analysis for detection of a halides sulphites, oxalates etc.

v) Bleaching of wool, cotton, silk and other textiles.

vi) Decolourization of oils

vii) In dry cells

viii) In dry cells for water  as being oxidant kills bacteria and doesn’t alter the taste of it unlike Cl ₂ which although disinfect but has an pleasant test.

KMnO ₄ is used as an intermediate solution in the titrations e.g.., quantitative estimation of Mn, iodine, thiosulphate etc.  But it cannot be used as primary standard, because of following reason: