KMnO4 (Potassium Permanganate): Uses, Structure, Preparation, Properties

Potassium permanganate is an odorless and purple to magenta crystalline solid. KMnO₄ is soluble in water, acetone, acetic acid, methanol, and pyridine. It easily dissolves in ethanol and organic solvents. Potassium permanganate is in the form of monoclinic prisms that is almost opaque with a blue metallic luster. When potassium permanganate crystals are dissolved in water the solution formed is purple. It is considered as a strong oxidizing agent and does not produce toxic by-products. It is usually prepared from other minerals such as manganese oxide. This compound features an ionic bond between the potassium cation and the permanganate anion.

Potassium permanganate is commercially prepared by mixing KOH and powdered manganese oxide, with oxidizing agents like potassium chlorate. The mixture is boiled and evaporated and the residue is heated in iron pans until it has acquired a pasty consistency. The potassium manganate (green) so formed is boiled with a large quantity of water and a current of chlorine CO₂ and ozonized air is passed into the liquid until it is converted into permanganate. The MnO₂ formed is removed continuously in order to prevent its breaking down the permanganate.

KMnO4 - Potassium Permanganate

Potassium permanganate is prepared by the fusion of MnO₂ with an alkali metal hydroxide and an oxidizing agent like KNO₃. This produces the dark green K₂MnO₄ which disproportionates in a neutral or acidic solution to give permanganate.

$2 \mathrm{MnO}_2+4 \mathrm{KOH}+\mathrm{O}_2 \rightarrow 2 \mathrm{~K}_2 \mathrm{MnO}_4+2 \mathrm{H}_2 \mathrm{O}$

$3 \mathrm{MnO}_4^{2-}+4 \mathrm{H}^{+}+\mathrm{O}_2 \rightarrow 2 \mathrm{MnO}_4^{-}+\mathrm{MnO}_2+2 \mathrm{H}_2 \mathrm{O}$

Commercially it is prepared by the alkaline oxidative fusion of MnO₂ followed by the electrolytic oxidation of manganate (Vl).

In the laboratory, manganese (II) ion salt is oxidized by peroxodisulphate to permanganate.

$2 \mathrm{Mn}^{2+}+5 \mathrm{~S}_2 \mathrm{O}_8^{2-}+8 \mathrm{H}_2 \mathrm{O} \rightarrow 2 \mathrm{MnO}_4+10 \mathrm{SO}_4^{2-}+16 \mathrm{H}^{-}$

Potassium permanganate forms dark purple (almost black) crystals which are isostructural with those of KClO₄. The salt is not very soluble in water (6.4 g/100 g of water at 293 K), but when heated it decomposes at 513 K.

$2 \mathrm{KMnO}_4 \rightarrow \mathrm{K}_2 \mathrm{MnO}_4+\mathrm{MnO}_2+\mathrm{O}_2$

It has two physical properties of considerable interest: its intense color and its diamagnetism along with temperature-dependent weak paramagnetism. These can be explained by the use of molecular orbital theory which is beyond the present scope.
The manganate and permanganate ions are tetrahedral; the π-bonding takes place by an overlap of the p orbitals of oxygen with the d orbitals of manganese. The green manganate is paramagnetic because of one unpaired electron but the permanganate is diamagnetic due to the absence of unpaired electron.

A few important oxidizing reactions of KMnO₄ are given below:
(1) In acidic solutions:

• Iodine is liberated from potassium iodide:$10 \mathrm{I}^{-}+2 \mathrm{MnO}_4^{-}+16 \mathrm{H}^{+} \rightarrow 2 \mathrm{Mn}^{2+}+8 \mathrm{H}_2 \mathrm{O}+5 \mathrm{I}_2$
• Fe²⁺ ion (green) is converted to Fe³⁺ (yellow):$5 \mathrm{Fe}^{2+}+\mathrm{MnO}_4^{-}+8 \mathrm{H}^{+} \rightarrow \mathrm{Mn}^{2+}+4 \mathrm{H}_2 \mathrm{O}+5 \mathrm{Fe}^{3+}$

(2) In neutral or faintly alkaline solutions:

• A notable reaction is the oxidation of iodide to iodate:$2 \mathrm{MnO}_4^{-}+\mathrm{H}_2 \mathrm{O}+\mathrm{I}^{-} \longrightarrow 2 \mathrm{MnO}_2+2 \mathrm{OH}^{-}+\mathrm{IO}_3^{-}$
• Thiosulphate is oxidized almost quantitatively to sulfate:$8 \mathrm{MnO}_4^{-}+3 \mathrm{~S}_2 \mathrm{O}_3^{2-}+\mathrm{H}_2 \mathrm{O} \longrightarrow 8 \mathrm{MnO}_2+6 \mathrm{SO}_4^{2-}+2 \mathrm{OH}^{-}$

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Some Solved Examples

Q.1 If $\mathrm{KMnO}_4$ is reduced by oxalic acid in an acidic medium, then oxidation number of Mn changes from-

(1) 4 to 2

(2) 6 to 4

(3) 7 to 2

(4) 7 to 4

Solution:

As we learned -

Oxalic acid reacts with acidified $\mathrm{KMnO}_4$ and gets oxidized to CO₂ reducing the permanganate ions to Mn²⁺. The reaction is given as

$2 \mathrm{KMnO}_4+3 \mathrm{H}_2 \mathrm{SO}_4+5(\mathrm{COOH})_2 \longrightarrow \mathrm{K}_2 \mathrm{SO}_4+2 \mathrm{MnSO}_4+8 \mathrm{H}_2 \mathrm{O}+10 \mathrm{CO}_2$

So, the oxidation state of Manganese changes from +7 to +2.

Hence, the answer is the option (3).

Q.2 Consider the following reactions:

$\mathrm{NaCl}+\mathrm{K}_2 \mathrm{Cr}_2 \mathrm{O}_7+\mathrm{H}_2 \mathrm{SO}_4$ (conc.) $\rightarrow A+$ side products

$A+\mathrm{NaOH} \rightarrow B+$ side products

$\mathrm{B}+\mathrm{H}_2 \mathrm{SO}_4($ dil. $)+\mathrm{H}_2 \mathrm{O}_2 \rightarrow C+$ side products

The sum of the total number of atoms in one molecule each of A, B, and C is:

(1) 18

(2) 16

(3) 14

(4) 20

Solution:

As we learned -

(A) = CrO₂Cl₂

(B) = Na₂CrO₄

(C) = CrO₅

Total atoms = 5 (A) + 7 (B) + 6 (C) = 18 atoms

Hence, the answer is the option(1).

Conclusion

There are wide applications of KMnO4. Some important uses of potassium permanganate are used in qualitative analysis to determine permanganate value. It is also used as a regeneration chemical in well water treatment for the removal of hydrogen sulfide and iron. This compound is used as a disinfectant to cure certain skin conditions like foot fungal infections, and dermatitis. The antibacterial action of KMnO4 is dependent on the oxidation of proteins of bacteria or tissues by this compound. It leaves a stain on the skin or tissues. Since it acts by a destructive oxidation process on all organic matter, its use is restricted for external purposes only