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Inorganic Chemistry

Potassium Permanganate ($KMnO_4$)

Preparation from Pyrolusite, Structure, and Oxidizing Action in Different Media.

By chemca Team • Updated Jan 2026

Potassium Permanganate is a deep purple crystalline solid. It is a powerful oxidizing agent used in the laboratory and industry. It is prepared from the black oxide ore of manganese, Pyrolusite ($MnO_2$).

1. Preparation from Pyrolusite Ore

Step 1: Conversion to Potassium Manganate

Pyrolusite ore ($MnO_2$) is fused with Potassium Hydroxide ($KOH$) in the presence of air ($O_2$) or an oxidizing agent like $KNO_3$ to form the green Potassium Manganate.

$$ 2MnO_2 + 4KOH + O_2 \xrightarrow{\Delta} \underset{\text{Potassium Manganate (Green)}}{2K_2MnO_4} + 2H_2O $$

Step 2: Oxidation to Potassium Permanganate

The green mass is extracted with water and oxidized to permanganate. This can be done chemically (using $Cl_2$, $O_3$, or $CO_2$) or Electrolytically.

A. Electrolytic Oxidation (Modern Method):

$$ K_2MnO_4 \rightleftharpoons 2K^+ + MnO_4^{2-} $$ Anode: $$ MnO_4^{2-} \rightarrow \underset{\text{Purple}}{MnO_4^-} + e^- $$

B. Disproportionation (Acidic/Neutral):

$$ 3MnO_4^{2-} + 4H^+ \rightarrow \underset{\text{Purple}}{2MnO_4^-} + \underset{\text{Black ppt}}{MnO_2} + 2H_2O $$

2. Structure and Physical Properties

Manganate Ion ($MnO_4^{2-}$)

• Color: Green
• Magnetism: Paramagnetic (1 unpaired electron)
• Geometry: Tetrahedral

Permanganate Ion ($MnO_4^-$)

• Color: Deep Purple
• Magnetism: Diamagnetic (No unpaired electrons)
• Geometry: Tetrahedral ($d^3s$ hybridisation involving d-orbitals from higher shell is incorrect concept, actually Charge Transfer spectrum causes color)

Action of Heat: On heating at 513 K, it decomposes to give Oxygen.

$$ 2KMnO_4 \xrightarrow{513K} K_2MnO_4 + MnO_2 + O_2 $$

3. Oxidizing Properties

KMnO4 acts as a strong oxidizing agent in acidic, neutral, and basic media.

A. In Acidic Medium ($pH < 7$)

Reduced to Manganous ion ($Mn^{2+}$).
Change in O.S.: $+7 \rightarrow +2$ (n-factor = 5).

$$ MnO_4^- + 8H^+ + 5e^- \rightarrow Mn^{2+} + 4H_2O $$

Important Reactions:

• $Fe^{2+}$ (Green) $\rightarrow$ $Fe^{3+}$ (Yellow)
• $I^-$ (Iodide) $\rightarrow$ $I_2$ (Iodine)
• $C_2O_4^{2-}$ (Oxalate) $\rightarrow$ $CO_2$ (at 333 K)
• $H_2S \rightarrow S$ (Sulphur)
• $SO_3^{2-} \rightarrow SO_4^{2-}$
• $NO_2^- \rightarrow NO_3^-$

B. In Neutral or Faintly Alkaline Medium

Reduced to Manganese Dioxide ($MnO_2$).
Change in O.S.: $+7 \rightarrow +4$ (n-factor = 3).

$$ MnO_4^- + 2H_2O + 3e^- \rightarrow \underset{\text{Brown ppt}}{MnO_2} + 4OH^- $$

Important Reactions:

• $I^-$ (Iodide) $\rightarrow$ $IO_3^-$ (Iodate) (Contrast with acidic medium)
• $S_2O_3^{2-}$ (Thiosulphate) $\rightarrow$ $SO_4^{2-}$ (Sulphate)

C. In Strongly Alkaline Medium

Reduced to Manganate ion ($MnO_4^{2-}$).
Change in O.S.: $+7 \rightarrow +6$ (n-factor = 1).

$$ MnO_4^- + e^- \rightarrow \underset{\text{Green}}{MnO_4^{2-}} $$

Baeyer's Reagent: Cold, dilute, alkaline KMnO4 solution is used to test for unsaturation (alkenes/alkynes). The purple color disappears, and a brown ppt of $MnO_2$ forms.

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