Potassium permanganate

Potassium permanganate - KMnO₄

Preparation:

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)

Mn₂O₇ →^Δ→ 2MnO₂ + 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.