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Preparation, Properties, Structure, Uses - Potassium permanganate | 12th Chemistry : UNIT 4 : Transition and Inner Transition Elements

Chapter: 12th Chemistry : UNIT 4 : Transition and Inner Transition Elements

Potassium permanganate

Potassium permanganate exists in the form of dark purple crystals which melts at 513 K.

Potassium permanganate - KMnO4



Potassium permanganate is prepared from pyrolusite (MnO2) ore. The preparation involves the following steps.

i) Conversion of MnO2 to potassium manganate:

Powdered ore is fused with KOH in the presence of air or oxidising agents like KNO3 or KClO3. A green coloured potassium manganate is formed.

2MnO2 + 2KOH + O2 → K2MnO4 + 2H2O

K2MnO4  = potassium manganate (Green)

ii) Oxidation of potassium manganate to potassium permanganate: Potassium manganate thus obtained can be oxidised in two ways , either by chemical oxidation or electrolytic oxidation.


Chemical oxidation:

In this method potassium manganate is treated with ozone (O3) or chlorine to get potassium permanganate.

2MnO42 + O3 + H2O → 2MnO4 + 2OH + O2

2MnO42 + Cl2 → 2MnO4 + 2Cl


Electrolytic oxidation

In this method aqueous solution of potassium manganate is electrolyzed in the presence of little alkali.

K2MnO4 ↔ 2K+ + MnO42

H2O ↔ H+ + OH

Manganate ions are converted into permanganate ions at anode.

MnO42 ↔ MnO4 + e

Green purple

H2is liberated at the cathode.

2H+ + 2e → H2

The purple coloured solution is concentrated by evaporation and forms crystals of potassium permanganate on cooling.


Physical properties:

Potassium permanganate exists in the form of dark purple crystals which melts at 513 K.

It is sparingly soluble in cold water but, fairly soluble in hot water.


Structure of permanganate ion

Permanganate ion has tetrahedral geometry in which the central Mn7+ is sp3 hybridised.


Chemical properties:

1. Action of heat:

When heated, potassium permanganate decomposes to form potassium manganate and manganese dioxide.

2KMnO4 → 2K2MnO4 + MnO2 + O2

2. Action of conc H2SO4

On treating with cold conc H2SO4, it decomposes to form manganese heptoxide, which subsequently decomposes explosively.

2KMnO4 + 2H2SO4 (cold) → Mn2O7 + 2KHSO4 + H2O (cold)

Mn2O7Δ2MnO2 + 3O2

But with hot conc H2SO4, potassium permanganate give MnSO4

4KMnO4 + 6H2SO4(hot) → 4MnSO4 + 2K2SO4 + 6H2O + 5O2 (hot)

3. Oxidising property:

Potassium permanganate is a strong oxidising agent, its oxidising action differs in different reaction medium.

a) In neutral medium:

In neutral medium, it is reduced to MnO2.

MnO4 + 2H2O + 3e → MnO2 + 4OH

(i) It oxidises H2S to sulphur

2MnO4 + 3H2S → 2MnO2 + 3S + 2OH + 2H2O

(ii) It oxidises thiosulphate into sulphate

8MnO4 + 3S2O32 + H2O → 6SO42 + 8MnO2 + 2OH

b) In alkaline medium:

In the presence of alkali metal hydroxides, the permanganate ion is converted into manganate.

MnO4 + e → MnO42

This manganate is further reduced to MnO2 by some reducing agents.

MnO42 + H2O → MnO2 + 2OH + [O]

So the overall reaction can be written as follows.

MnO4 + 2H2O + 3e → MnO2 + 4OH

This reaction is similar as that for neutral medium.

Bayer’s reagent:

Cold dilute alkaline KMnO4 is known as Bayer’s reagent. It is used to oxidise alkenes into diols. For example, ethylene can be converted into ethylene glycol and this reaction is used as a test for unsaturation.

c) In acid medium:

In the presence of dilute sulphuric acid, potassium permanganate acts as a very strong oxidising agent. Permanganate ions is converted into Mn2+ ion.

MnO4 + 8H+ + 5e → Mn2+ + 4H2O

The oxidising nature of potassium permanganate (permanganate ion) in acid medium is illustrated in the following examples.

(i) It oxidises ferrous salts to ferric salts.

2MnO4 + 10Fe2+ + 16H+ → 2Mn2+ + 10Fe3+ + 8H2O

(ii) It oxidises iodide ions to iodine

2MnO4 + 10I + 16H+ → 2Mn2+ + 5I2 + 8H2O

(iii) It oxidises oxalic acid to CO2

2MnO4 + 5( COO)2 + 16H+ → 2Mn2+ + 10CO2 + 8H2O

(iv) It oxidises sulphide ion to sulphur

2MnO4 + 5 S2 + 16H+ → 2Mn2+ + 5 S + 8H2O

(vi) It oxidises nitrites to nitrates

2MnO4 + 5NO2 + 6H+ → 2Mn2+ + 5NO3 + 3H2O

(vi) It oxidises alcohols to aldehydes.

2KMnO4 + 3H2SO4 + 5CH3CH2OH → 2K2SO4 + 2MnSO4 + 5CH3CHO + 8H2O

(vii) It oxidises sulphite to sulphate

2MnO4 + 5SO32 + 6H+ → 2Mn2+ + 5SO42 + 3H2O


Uses of potassium permanganate:

Some important uses of potassium permanganate are listed below.

1.        It is used as a strong oxidizing agent.

2.     It is used for the treatment of various skin infections and fungal infections of the foot.

3.     It used in water treatment industries to remove iron and hydrogen sulphide from well water.

4.     It is used as a Bayer’s reagent for detecting unsaturation in an organic compound.

5.        It is used in quantitative analysis for the estimation of ferrous salts, oxalates, hydrogen peroxide and iodides.

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