Potassium Permanganate

Potassium Permanganate, KMnO₄ 

It is prepared from mineral pyrolusite (MnO₂) by following steps :

1) Conversion of Pyrolusite ore to Potassium Manganate

The pyrolusite is fused with caustic soda or potassium carbonate in the presence of air or oxidising agent such as potassium nitrate or potassium chlorate to give a green mass due to the formation of potassium manganate.

2 MnO₂ + 4KOH + O₂ → 2K₂MnO₄ + 2 H₂O

2 MnO₂ + 2 K₂CO₃ + O₂ → 2 K₂MnO₄ + 2CO₂

MnO₂ + 2 KOH + KNO₃ → K₂MnO₄ + KNO₂ + H₂O

2) Oxidation of Potassium Manganate to Potassium Permanganate

The green mass is extracted with water resulting in green solution of potassium manganate. The solution is then , treated with a current of chlorine or ozone or carbon dioxide to oxidise potassium manganate to potassium permanganate. The solution is concentrated and dark purple crystals of potassium permanganate separate out.

2 K₂MnO₄ + Cl₂ → 2 KCl + 2 KMNO₄

2 K₂MnO₄ + O₃ + H₂O → 2 KMnO₄ + 2 KOH + O₂ 

3K₂MnO₄ + 2CO₂ → 2K₂CO₃ + 2MnO₂ + 2 KMnO₄

Manganate ion also disproportionate in a neutral or acidic solution to give permanganate

3 MnO₄^2- + 4 H⁺ → 2 MnO₄¯ + MnO₂ + 2H₂O

The alkaline potassium manganate solution is electrolytically oxidised.

Electrolytic Method

Potassium permanganate is prepared by alkaline oxidative fusion of MnO₂ followed by electrolytic oxidation of manganate (VI).

The potassium manganate solution is taken in an electrolytic cell which contains iron cathode and nickel anode.

The Potassium manganate solution is taken in anodic compartment while dilute solution is added in the cathode compartment.

When the current is passed the manganate ion is oxidised to permanganate ion at anode and hydrogen is liberated at cathode.

2 K₂MnO₄ ⇔ 2 K+ + MNO₄ ^2-

At anode

MNO₄ ^2- → MnO₄¯ + e-

(green)              (purple)

At cathode

2 H⁺ + 2 e → 2H

2H → H₂

Laboratory preparation

It is prepared by oxidising a manganese (II) ion salt by peroxodisulphide.

2Mn²⁺ + S₂O₈^2- + 8H₂O → 2MNO₄^– + 10 SO₄^2- + 16H⁺

Properties of Potassium Permanganate

1) Colour and melting point : It is a dark violet crystalline solid having a metallic lusture. It has m.p.523 K.

2) Solubility : It is fairly soluble in water giving a purple solution.

3) Action of heat : When heated strongly to 513 K , it decomposes to give oxygen.

2 KMnO₄ → K₂MNO₄ + MnO₂ + O₂

At red heat potassium manganate formed decomposes into potassium manganite and oxygen.

2 K₂MnO₄ → 2 K₂MNO₃ + O₂

4) Action of alkalies

On heating with alkalies, potassium permanganate changes into potassium manganate  and oxygen gas is evolved.

4KMnO₄ + 4KOH → 4K₂MnO₄ +2 H₂O + O₂

5) Oxidising character

It is a powerful oxidising agent in neutral , alkaline or acidic solution because it liberates nascent oxygen

In Neutral solution MnO₂ is formed

2 KMnO₄ + H₂O → 2 KOH + 2 MNO₄ + 3O

MnO₄¯ + 2 H₂O + 3 e¯ → MnO₂ + 4 OH‾

In strongly alkaline solution manganate ions are formed

2 KMnO₄ + H₂O  → 2 K₂MnO₄ + H₂O + O

MnO₄¯ + e‾ → MnO₄^2-

Eq. wt. of KMnO₄ = Mol wt. / 1 = 158 /1 =158

Potassium manganate is further reduced to MNO₂ in the presence of a reducing agent.

K₂MnO₄ + H₂O → MnO₂ + 2 KOH + O ] × 2

MnO₄^2- + 2 H₂O + 2 e¯ → MnO₂ + 4 OH¯

2 KMnO₄ + H₂O → 2 MNO₂ + 2 KOH + 3O

2 MNO₄¯ + 2 H₂O + 3 e‾ → MnO₂ + 4OH¯

In Acidic solution

Mn²⁺ ions are formed

2 KMnO₄ + 3 H₂SO₄ → K₂SO₄ + 2 MnSO₄ + 3 H₂O + 5 O

MnO₄¯ + 8H+ + 5e¯ → Mn²⁺ + 4 H₂O

The oxidation reactions of acidified KMnO₄ are :

1) It oxidises acidified ferrous salts (green) to ferric salts (yellow)

2 KMnO₄ + 3 H₂SO₄ → K₂SO₄ + 2 MnSO₄ + 3 H₂O + 5 O

2 FeSO₄ + H₂SO₄ +O → Fe₂(SO₄)₃ + H₂O ] × 5

______________________________

2 KMnO₄ + 10  FeSO₄ + 8 H₂SO₄ → K₂SO₄ + 2 MnSO₄ + 5 Fe₂(SO₄)₃ + 8 H₂O

______________________________

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

2) It oxidises hydrogen sulphide to sulphur

2 KMnO₄ + 3 H₂SO₄ → K₂SO₄ + 2 MnSO₄ + 3 H₂O + 5 O

H₂S + O → H₂S + S ] × 5

______________________________

2 KMnO₄ + 5 H₂S + 3 H₂SO₄ → K₂SO₄ + 2 MnSO₄ + 5 S + 8 H₂O

______________________________

or 2 MnO₄¯+ 5S^2- + 16 H⁺ → 2 Mn²⁺ + 5S+ 8 H₂O

3) It oxidises acidified potassium iodide to iodine

2 KMnO₄ + 3 H₂SO₄ → K₂SO₄ + 2 MnSO₄ + 3 H₂O + 5 O

2 KI + H₂SO₄ + O → K₂SO₄ + H₂O + I₂ ] × 5

______________________________

10KI + 2 KMnO₄ + 8 H₂SO₄ → 6 K₂SO₄ + 2 MnSO₄ + 8 H₂O + 5 I₂

______________________________

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

 

4) It oxidises sulphur dioxide to sulphuric acid

2 KMnO₄ + 3 H₂SO₄ → K₂SO₄ + 2 MnSO₄ + 3 H₂O + 5 O

SO₂ + H₂O + O → H₂SO₄ ] × 5

______________________________

2 KMnO₄ + 5SO₂ + 2 H₂O → K₂SO₄ + 2 MnSO₄ + 2 H₂SO₄ 

______________________________

or 2MnO₄ ¯ + 5 SO₂ + 2 H₂O → 5 SO₄^2– + 2 Mn²⁺ + 4 H⁺

 

5) It oxidises nitrites to nitrates

2 KMnO₄ + 3 H₂SO₄ → K₂SO₄ + 2 MnSO₄ + 3 H₂O + 5 O

KNO₂ + O → KNO₃ ] × 5

______________________________

2 KMnO₄ + 5 KNO₂ + 3 H₂SO₄ → K₂SO₄ + 2 MnSO₄ + 5 KNO₃ + 3 H₂O

______________________________

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

6) It oxidises oxalates or oxalic acid to carbon dioxide

2 KMnO₄ + 3 H₂SO₄ → K₂SO₄ + 2 MnSO₄ + 3 H₂O + 5 O

C₂H₂O₄ + O → 2 CO₂ + H₂O ] × 5

______________________________

2 KMnO₄ + 5 C₂H₂O₄ + 3 H₂SO₄ → K₂SO₄ + 2 MnSO₄ +  10 CO₂ + 8 H₂O

______________________________

or 2MnO₄¯ + 5 C₂O₄ ^2– + 16 H⁺ → 2 Mn²⁺ + 8 H₂O + 10 CO₂

 

7) It oxidises hydrogen halides to halogen

2 KMnO₄ + 3 H₂SO₄ → K₂SO₄ + 2 MnSO₄ + 3 H₂O + 5 O

2 HX + O → H₂O + X₂ ] × 5

______________________________

2 KMnO₄ + 10 HX + 3 H₂SO₄ → K₂SO₄ + 2 MnSO₄ + 8 H₂O + 5 X₂

______________________________

or 2MnO₄¯ + 10 X¯ + 6 H⁺ → 2 Mn²⁺ + 8 H₂O + 5 X₂

 

8) It oxidises sulphites or sulphurous acid to sulphates or sulphuric acid

2 KMnO₄ + 3 H₂SO₄ → K₂SO₄ + 2 MnSO₄ + 3 H₂O + 5 O

Na₂SO₃ + O → Na₂SO₄ ] × 5

______________________________

2 KMnO₄ + 3 H₂SO₄ + 5 Na₂SO₃ → K₂SO₄ + 2 MnSO₄ + 5 Na₂SO₄ + 3 H₂O

______________________________

or 2 MnO₄¯ + 5 SO₃^2- + 6 H⁺ → 2 Mn²⁺ + 5 SO₄^2- + 3 H₂O

In Neutral Medium

1) It oxidises hydrogen sulphide to sulphur

2 KMnO₄ + H₂O → 2 KOH + 2 MnO₂ + 3 O

H₂S + O → H₂S + S ] × 3

______________________________

2 KMnO₄ + 3 H₂S → 2 KOH + 2 MnO₂ + 2 H₂O + 3 S

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

2) It oxidises manganese sulphate to manganese dioxide

2 KMnO₄ + H₂O → 2 KOH + 2 MnO₂ + 3 O

MnSO₄ + H₂O + O → MnO₂ + H₂SO₄ ] × 3

2KOH + H₂SO_{4 →} K₂SO₄ + 2 H₂O

______________________________

2 KMnO₄ +3  MnSO₄ + 2 H₂O → 5 MnO₂ + K₂SO₄ + 2 H₂SO₄

______________________________

or 2 MnO₄^– + 3 Mn²⁺ + 2 H₂O → 5 MnO₂ + 4 H⁺

3) It oxidises sodium thiosulphate to sodium sulphate or sulphur.

8 KMnO₄ + 3 Na₂S₂O₃ + H₂O → 8 MnO₂ + 3 Na₂SO₄ + 2 KOH + 3 K₂SO₄

or 8 MnO₄¯ + 3 S₂O₃^2- + H₂O → 8 MnO₂ + 6 SO₄^2- + 2 OH¯

In Alkaline Medium

1) It oxidises iodides to iodates in alkaline solution

2 KMnO₄ + H₂O → 8 MnO₂ + 2 KOH + 3 O

KI + 3O → KIO₃

______________________________

2 KMnO_{4 +} KI + H₂O → 2 MNO₂ + 2 KOH + KIO₃

______________________________

2 MnO₄¯ + I¯ + H₂O → 2 MnO₂ + IO₃¯ + 2 OH¯

2) Alkaline KMnO4 oxidises ethylene to ethylene glycol

C₂H₄ + H₂O + O → C₂H₄(OH)₂

Uses of Potassium Permanganate

1) It is used as an oxidising agent in the laboratory in volumetric analysis for the estimation of ferrous salts , oxalates , iodides and hydrogen peroxide.

2) It is used as a disinfectant and germicide. It is used to purify well water.

3) It is used in industry as a strong oxidising agent.

4) Alkaline potassium permanganate is used in organic chemistry under the name Baeyer’s reagent.

5) It is used for the bleaching of wool, cotton , silk and other textile fibres.

6) It is used for decolorisation of oils because of its strong oxidising agent.

Structure of MnO₄¯and MnO₄^2- ion

Permanganate and manganate ion have tetrahedral shape. The four atoms are arranged tetrahedrally around manganese. The pi bonding takes place by the overlap of p-orbital of oxygen with d-orbital of manganese.

Important Note

1) Potassium permanganate is not a primary standard because it is not available in the pure state and contain traces of MnO₂. It is therefore standardised with a standard solution of oxalic acid or sodium oxalate.

2) Potassium permanganate titrations are carried out only in the presence of dil H₂SO₄ because oxygen produced from the reaction of KMnO₄ with dil H₂SO₄ is used only for oxidising the reducing agent.

3) HCl or HNO₃ can not be used in place of H₂SO₄ .

a) If HCl is used , the oxygen produced by the reaction of KMnO₄ and dil HCl will be partially used to oxidise HCl to chlorine.

b) If HNO₃ is used , it itself acts as an oxidising agent and will partially oxidize the reducing agent.