Properties of Potassium Permanganate | Uses & Reactions

Properties of Potassium Permanganate (KMnO₄)

Potassium permanganate is one of the most widely used oxidising agents in both inorganic and organic chemistry. It is a salt of permanganic acid (HMnO₄) in which manganese exists in its highest oxidation state of +7. Its strong oxidising power, distinctive deep purple colour, and versatility across acidic, neutral, and alkaline media make it an indispensable reagent in qualitative and quantitative analysis.

Physical Properties of Potassium Permanganate (KMnO₄)

Appearance and colour: KMnO₄ is a dark purple or violet-black crystalline solid. Its intense colour arises due to electronic transitions in MnO₄⁻ involving the permanganate ion (MnO₄⁻), where electron density is transferred from oxygen to manganese under the influence of light.

Solubility: It is moderately soluble in water, producing a characteristic deep purple solution. The colour of the solution is so intense that even very dilute solutions appear strongly coloured.

Melting point: KMnO₄ decomposes on heating at approximately 513 K rather than melting cleanly, releasing oxygen in the process:

2KMnO₄ → K₂MnO₄ + MnO₂ + O₂

Density and stability: It is a dense, stable crystalline solid under normal conditions, but is a strong oxidising agent that reacts vigorously with reducing agents and organic matter.

Chemical Properties of Potassium Permanganate (KMnO₄)

Oxidising Action in Acidic Medium

In acidic medium (dilute H₂SO₄), MnO₄⁻ is reduced from Mn(+7) to Mn(+2), a change of 5 electrons per manganese atom. The solution changes from deep purple to nearly colourless as Mn²⁺ is formed.

Half reaction:

MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O

Oxidising Action in Neutral or Faintly Alkaline Medium

In neutral or faintly alkaline medium, MnO₄⁻ is reduced to MnO₂, a change of 3 electrons per manganese atom. A brown precipitate of MnO₂ is formed.

Half reaction:

MnO₄⁻ + 2H₂O + 3e⁻ → MnO₂ + 4OH⁻

Oxidising Action in Strongly Alkaline Medium

In a strongly alkaline medium, MnO₄⁻ is reduced to manganate ion MnO₄²⁻, a change of only 1 electron per manganese atom. The solution turns green due to the formation of MnO₄²⁻.

Half reaction:

MnO₄⁻ + e⁻ → MnO₄²⁻

Summary of Oxidising Behaviour

Summary of Oxidising Behaviour
Medium	Product of Mn	Colour Change	Electrons Gained
Acidic	Mn²⁺	Purple to colourless	5
Neutral/Faintly alkaline	MnO₂	Purple to brown ppt	3
Strongly alkaline	MnO₄²⁻	Purple to green	1

Preparation of KMnO₄

KMnO₄ is prepared from pyrolusite ore (MnO₂) in two steps:

Step 1 - Fusion with KOH in the presence of air or KNO₃

2MnO₂ + 4KOH + O₂ → 2K₂MnO₄ + 2H₂O

Green potassium manganate (K₂MnO₄) is formed.

Step 2 - Oxidation of manganate to permanganate

Electrolytic oxidation:

2K₂MnO₄ + 2H₂O → 4KOH + 2KMnO₄ + H₂ (electrolysis)

Or by passing Cl₂:

2K₂MnO₄ + Cl₂ → 2KMnO₄ + 2KCl

Uses of KMnO₄

Used as a volumetric reagent in permanganometry for estimating reducing agents such as Fe²⁺, C₂O₄²⁻, and H₂O₂
Used as a disinfectant and germicide in a dilute solution
Used for bleaching wool, cotton, and silk
Used in organic chemistry for oxidative cleavage of alkenes and oxidation of primary alcohols to carboxylic acids
Used in water treatment for the removal of iron, manganese, and organic contaminants

Conclusion

Potassium permanganate is a versatile oxidising agent in which manganese exists in the +7 oxidation state. Its oxidising power varies with the medium; it gains 5 electrons in an acidic medium forming Mn²⁺, 3 electrons in a neutral medium forming MnO₂, and 1 electron in a strongly alkaline medium forming MnO₄²⁻.

It is prepared from MnO₂ by fusion with KOH, followed by electrolytic oxidation of manganate. Its wide applicability in titrimetry, organic synthesis, and disinfection makes it one of the most important inorganic reagents studied at this level.

FAQs

Q1. Why does KMnO₄ act as a self-indicator in permanganometry?

The permanganate ion imparts an intense purple colour even at very low concentrations. A single excess drop beyond the equivalence point gives a persistent pink or purple tint to the colourless solution, signalling the endpoint without requiring an external indicator.

Q2. Why does the oxidising power of KMnO₄ decrease in an alkaline medium compared to an acidic medium?

In an acidic medium, high H⁺ concentration facilitates a larger reduction in oxidation state, from +7 to +2, releasing more energy. In alkaline medium, limited H⁺ availability restricts the extent of reduction to +6 (manganate), lowering the electrode potential and thus the effective oxidising power.

Q3. Why is KMnO₄ stored in dark coloured bottles?

Exposure to light accelerates the decomposition of KMnO₄ into MnO₂, K₂MnO₄, and O₂. Dark bottles prevent light from reaching the reagent, preserving its concentration and oxidising strength over time.