Potassium Permanganate Formula: Structure, Preparation, Properties, Uses

Potassium Permanganate Formula: Potassium permanganate is an inorganic compound. Its chemical formula is \({\rm{KMn}}{{\rm{O}}_4}\). It is also known as permanganate of potash. In chemistry, it is famous as an oxidizing agent. In this article, we are going to discuss more about potassium permanganate \(\left( {{\rm{KMn}}{{\rm{O}}_4}} \right)\).

Structure of Potassium Permanganate

Potassium permanganate \(\left( {{\rm{KMn}}{{\rm{O}}_4}} \right)\) contains potassium \(\left( {{{\rm{K}}^ + }} \right)\) ions and permanganate \(\left( {{\rm{MnO}}_4^ – } \right)\) ions. There is an ionic bond between the potassium \(\left( {{{\rm{K}}^ + }} \right)\) ion and permanganate \(\left( {{\rm{MnO}}_4^ – } \right)\) ion. Potassium permanganate can be represented as,

The permanganate ion in potassium permanganate has a tetrahedral structure.

Formula of Potassium Permanganate

The formula of Potassium Permanganate is KMnO₄.

Preparation of Potassium Permanganate

1. Potassium permanganate can be prepared from pyrolusite \(\left( {{\rm{Mn}}{{\rm{O}}_2}} \right)\) with the help of following processes.

Step (i): Conversion of \({\rm{Mn}}{{\rm{O}}_2}\) into potassium manganate \(\left( {{{\rm{K}}_2}{\rm{Mn}}{{\rm{O}}_4}} \right)\):

In this process, the pyrolusite is fused with caustic potash \(({\rm{KOH}})\) or \({{\rm{K}}_2}{\rm{C}}{{\rm{O}}_3}\) in the presence of oxidizing agents like potassium nitrate or potassium chlorate \(\left( {{\rm{KCl}}{{\rm{O}}_3}} \right)\) in a reverberatory furnace. A green mass of \({{\rm{K}}_2}{\rm{Mn}}{{\rm{O}}_4}\) is obtained here. The chemical equation for the reaction can be given as,

\({\rm{KN}}{{\rm{O}}_3} \to {\rm{KN}}{{\rm{O}}_2} + {\rm{O}}\)

\({\rm{Mn}}{{\rm{O}}_2} + {\rm{KOH}} + {\rm{O}} \to {{\rm{K}}_2}{\rm{Mn}}{{\rm{O}}_4}{\rm{ (green) }} + {{\rm{H}}_2}{\rm{O}}\)

\(2{\rm{Mn}}{{\rm{O}}_2} + 2\;{{\rm{K}}_2}{\rm{C}}{{\rm{O}}_3} + {{\rm{O}}_2} \to 2\;{{\rm{K}}_2}{\rm{Mn}}{{\rm{O}}_4} + 2{\rm{C}}{{\rm{O}}_2}\)

Step (ii): Oxidation of \({{\rm{K}}_2}{\rm{Mn}}{{\rm{O}}_4}\) to \({\rm{KMn}}{{\rm{O}}_4}\):

In this step, the green mass of \({{\rm{K}}_2}{\rm{Mn}}{{\rm{O}}_4}\) is treated with water \(\left( {{{\rm{H}}_2}{\rm{O}}} \right)\) and then carbon dioxide \(\left( {{\rm{C}}{{\rm{O}}_2}} \right)\), chorine \(\left( {{\rm{C}}{{\rm{l}}_2}} \right)\) or ozone \(\left( {{{\rm{O}}_3}} \right)\) is passed through the solution. As a result, \({{\rm{K}}_2}{\rm{Mn}}{{\rm{O}}_4}\left( {{\rm{M}}{{\rm{n}}^{ + 6}}} \right)\) is oxidised to \({\rm{KMn}}{{\rm{O}}_4}\left( {{\rm{M}}{{\rm{n}}^{ + 7}}} \right)\). Hence, a pink solution is obtained. The chemical equations for the reactions can be given as,

The pink solution obtained during the reaction is filtered to remove insoluble manganese dioxide \(\left( {{\rm{Mn}}{{\rm{O}}_2}} \right)\), and then the filtrate is concentrated to get pure crystals of potassium permanganate \(\left( {{\rm{KMn}}{{\rm{O}}_4}} \right)\).

2. Potassium permanganate can also be prepared from potassium manganate \(\left( {{{\rm{K}}_2}{\rm{Mn}}{{\rm{O}}_4}} \right)\) by the action of dilute sulphuric acid \(\left( {{{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4}} \right)\) on it. That is,

\(3\;{{\rm{K}}_2}{\rm{Mn}}{{\rm{O}}_4} + 2{{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4} \to 2\;{{\rm{K}}_2}{\rm{S}}{{\rm{O}}_4} + 2{\rm{KMn}}{{\rm{O}}_4} + {\rm{Mn}}{{\rm{O}}_2} \downarrow + {{\rm{H}}_2}{\rm{O}}\)

The insoluble \({\rm{Mn}}{{\rm{O}}_2}\) obtained during the reaction is removed by filtration. Potassium permanganate \(\left( {{\rm{KMn}}{{\rm{O}}_4}} \right)\) formed here is less soluble than potassium sulphate \(\left( {{{\rm{K}}_2}{\rm{S}}{{\rm{O}}_4}} \right)\) formed here. Therefore, during crystallization, potassium permanganate \(\left( {{\rm{KMn}}{{\rm{O}}_4}} \right)\) crystallises first.

3. By the electrolysis of potassium manganate \(\left( {{{\rm{K}}_2}{\rm{Mn}}{{\rm{O}}_4}} \right)\):

Potassium permanganate can be prepared by the electrolysis of potassium manganate \(\left( {{{\rm{K}}_2}{\rm{Mn}}{{\rm{O}}_4}} \right)\). In this process, potassium manganate \(\left( {{{\rm{K}}_2}{\rm{Mn}}{{\rm{O}}_4}} \right)\) solution is taken in an electric tank having nickel as anode and iron as cathode. A porous partition separates the cathode from anode. When electricity is passed through the solution, the following reactions takes place at the respective electrodes. That is,

The \({{\rm{K}}^ + }\) ions which did not get discharged at cathode under the conditions of experiment react with \({\rm{MnO}}_4^ – \) to form \({\rm{KMn}}{{\rm{O}}_4}\). That is,

\({{\rm{K}}^ + } + {\rm{MnO}}_4^ – \to {\rm{KMn}}{{\rm{O}}_4}\)

The pink coloured solution of \({\rm{KMn}}{{\rm{O}}_4}\) obtained is now concentrated and cooled to get the pure crystals of potassium permanganate \(\left( {{\rm{KMn}}{{\rm{O}}_4}} \right)\).

4. The laboratory preparation of potassium permanganate involves the oxidation of a manganese (II) ion salt with potassium peroxodisulphate. That is,

\(2{\rm{M}}{{\rm{n}}^{2 + }} + 5\;{{\rm{S}}_2}{\rm{O}}_8^{2 – } + 8{{\rm{H}}_2}{\rm{O}} \to 2{\rm{MnO}}_4^ – + 10{\rm{SO}}_4^{2 – } + 16{{\rm{H}}^ + }\)

Properties of Potassium Permanganate (KMnO₄)

Let us discuss the physical and chemical properties of potassium permanganate in detail.

Physical Properties of Potassium Permangante

1. Potassium permanganate is a dark purple (almost black) crystal which is isostructural with \({\rm{KCl}}{{\rm{O}}_4}\). An aqueous solution of potassium permanganate appears pink.
2. Potassium permanganate is not much soluble in water. For instance, at \(293\;{\rm{K}}\), about \({\rm{6}}{\rm{.4}}\,{\rm{g}}\) of potassium permanganate dissolves in \(100\;{\rm{g}}\) of water. At higher temperature, it dissolves more.
3. It is affected by light. Therefore, it is kept in dark bottles.
4. It shows weak temperature-dependent paramagnetism.

Chemical Properties of Potassium Permangante

The chemical properties of Potassium Permanganate (KMnO₄) are explained below:

1. Action of Heat on Potassium Permanganate

When potassium permanganate is heated at \(473\;{\rm{K}}\), potassium manganate is formed. The chemical equation for the reaction can be given as,

\(2{\rm{KMn}}{{\rm{O}}_4} \to {{\rm{K}}_2}{\rm{Mn}}{{\rm{O}}_4} + {\rm{Mn}}{{\rm{O}}_2} + {{\rm{O}}_2}\)

On further heating to red heat, potassium manganate is converted into potassium manganite and oxygen. That is,

\(2\;{{\rm{K}}_2}{\rm{Mn}}{{\rm{O}}_4} \to 2\;{{\rm{K}}_2}{\rm{Mn}}{{\rm{O}}_3} + {{\rm{O}}_2}\)

2. Reaction of Potassium Permanganate with Concentrated Sulphuric Acid

A highly explosive substance \({\rm{M}}{{\rm{n}}_2}{{\rm{O}}_7}\) is formed when potassium permanganate is dissolved in concentrated sulphuric acid. That is,

\(2{\rm{KMn}}{{\rm{O}}_4} + {{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4} \to {{\rm{K}}_2}{\rm{S}}{{\rm{O}}_4} + {\rm{M}}{{\rm{n}}_2}{{\rm{O}}_7} + {{\rm{H}}_2}{\rm{O}}\)

\(2{\rm{M}}{{\rm{n}}_2}{{\rm{O}}_7}\,{\rm{Explodes}} \to 4{\rm{Mn}}{{\rm{O}}_2} + 3{{\rm{O}}_2}\)

3. Oxidising Nature Of Potassium Permanganate

Potassium permanganate is a powerful oxidizing agent in both acidic, alkaline, and neutral mediums due to the formation of nascent oxygen during the reaction. It can be explained in the following manner:

In Acidic Medium: \({\rm{M}}{{\rm{n}}^{2 + }}\) ions are formed.

\(2{\rm{KMn}}{{\rm{O}}_4} + 3{{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4} \to {{\rm{K}}_2}{\rm{S}}{{\rm{O}}_4} + 2{\rm{MnS}}{{\rm{O}}_4} + 3{{\rm{H}}_2}{\rm{O}} + 5{\rm{O}}\)

The examples showing the reactions of potassium permanganate in acidic medium can be given as,

i. Reactions in Acidic Medium

In acidic medium, it converts ferrous sulphate into ferric sulphate. That is,

In acidic medium, it converts oxalic acid into carbon dioxide. That is,

In acidic medium, it converts hydrochloric acid to chlorine. That is,

In Alkaline Medium: In alkaline medium, manganate \(\left( {{\rm{Mn}}{{\rm{O}}_4}^{2 – }} \right)\) ions are formed first, which then get reduced to \({\rm{Mn}}{{\rm{O}}_2}\) in the presence of reducing agents.

\(2{\rm{KMn}}{{\rm{O}}_4} + 2{\rm{KOH}} \to 2\;{{\rm{K}}_2}{\rm{Mn}}{{\rm{O}}_4} + {{\rm{H}}_2}{\rm{O}} + {\rm{O}}\)
\(\left. {{{\rm{K}}_2}{\rm{Mn}}{{\rm{O}}_4} + {{\rm{H}}_2}{\rm{O}} \to {\rm{Mn}}{{\rm{O}}_2} + 2{\rm{KOH}} + {\rm{O}}} \right] \times 2\)

The complete reaction is: \(2{\rm{KMn}}{{\rm{O}}_4} + {{\rm{H}}_2}{\rm{O}} \to 2{\rm{Mn}}{{\rm{O}}_2} + 2{\rm{KOH}} + 3{\rm{O}}\) or
\({\rm{MnO}}_4^ – + 2{{\rm{H}}_2}{\rm{O}} + 3{{\rm{e}}^ – } \to {\rm{Mn}}{{\rm{O}}_2} + 4{\rm{O}}{{\rm{H}}^ – }\)

The examples showing the reactions of potassium permanganate in alkaline medium can be given as,

ii. Reactions in Alkaline Medium

In basic medium, potassium permanganate converts manganese sulphate to manganese dioxide. That is,

In basic medium, potassium permanganate converts iodides to iodates. That is,

In basic medium, potassium permanganate converts thiosulphate to sulphate. That is,
\(8{\rm{Mn}}{{\rm{O}}^{4 – }} + 3\;{{\rm{S}}_3}{{\rm{O}}_3}^{2 – } + {{\rm{H}}_2}{\rm{O}} \to 8{\rm{Mn}}{{\rm{O}}_2} + 6{\rm{S}}{{\rm{O}}_4}^{2 – } + 2{\rm{O}}{{\rm{H}}^ – }\)

In Neutral Medium: In neutral medium, \({\rm{Mn}}{{\rm{O}}_2}\) is formed.

\(2{\rm{KMn}}{{\rm{O}}_4} + {{\rm{H}}_2}{\rm{O}} \to 2{\rm{KOH}} + 2{\rm{Mn}}{{\rm{O}}_2} + 3{\rm{O}}\) or

\({\rm{MnO}}_4^ – + 2{{\rm{H}}_2}{\rm{O}} + 3{{\rm{e}}^ – } \to {\rm{Mn}}{{\rm{O}}_2} + 4{\rm{O}}{{\rm{H}}^ – }\)

iii. Reactions in Neutral Medium

The reactions of potassium permanganate are almost same as in each case with faintly alkaline solutions.

For example, in neutral medium, \({{\rm{H}}_2}\;{\rm{S}}\) is oxidised to sulphur. That is,
\(2{\rm{MnO}}_4^ – + 3{{\rm{H}}_2}\;{\rm{S}} \to 2{\rm{Mn}}{{\rm{O}}_2} + 3\;{\rm{S}} + 2{{\rm{H}}_2}{\rm{O}} + 2{\rm{O}}{{\rm{H}}^ – }\)

Potassium permanganate oxidises thiosulphate to sulphate and sulphur. That is,
\(2{\rm{MnO}}_4^ – + 3\;{{\rm{S}}_2}{{\rm{O}}_3}^{2 – } + {{\rm{H}}_2}{\rm{O}} \to 2{\rm{Mn}}{{\rm{O}}_2} + 3{\rm{S}}{{\rm{O}}_4}^{2 – } + 3\;{\rm{S}} + 2{\rm{O}}{{\rm{H}}^ – }\)

Uses of Potassium Permanganate

Potassium permanganate has got so many uses. They are:

• 1. Potassium permanganate is used as an oxidizing agent in the volumetric analysis for the quantitative estimation of reducing agents like \({\rm{F}}{{\rm{e}}^{2 + }}\) ions, halides, oxalate, tartrates, sulphides, nitrites, etc.
• Note: Potassium permanganate titrations are not carried out in presence of hydrochloric acid \({\rm{ (HCl) }}\). This is because, \({\rm{KMn}}{{\rm{O}}_4}\) oxidises hydrochloric acid \({\rm{ (HCl) }}\) to \({\rm{C}}{{\rm{l}}_2}\).
• 2. In hospitals, it is used for gargling sore throat.
• 3. It is used as a disinfectant for the purification of water.
• 4. It is used in the laboratory as Baeyer’s reagent \(\left( {{\rm{KMn}}{{\rm{O}}_4} + {\rm{KOH}}} \right)\) to detect the presence of unsaturation in organic compounds.
• 5. It is used for making dry cells.

Summary

Potassium permanganate \(\left( {{\rm{KMn}}{{\rm{O}}_4}} \right)\) is a dark purple (almost black) inorganic chemical compound, and a well-known oxidizing agent. In this article, we have learnt about the chemical formula, structure, methods of preparation, physical and chemical properties, and the uses of potassium permanganate in different fields.

FAQs on Potassium Permanganate

Q.1. What is the formula of potassium permanganate?
Ans: Potassium permanganate is an inorganic compound. Its chemical formula is \({\rm{KMn}}{{\rm{O}}_4}\). It is a strong oxidizing agent.

Q.2. What is the formula unit mass of potassium permanganate?
Ans: Formula unit mass is the sum of the atomic masses of all individual atoms present in a formula unit of the given compound.
The atomic mass of potassium is \(39.0983\,{\rm{u}}.\)
The atomic mass of manganese is \(54.9380\,{\rm{u}}.\)
The atomic mass of oxygen is \(15.9990\,{\rm{u}}.\)
Therefore, the formula unit mass of potassium permanganate \(\left( {{\rm{KMn}}{{\rm{O}}_4}} \right) = \left( {1 \times 39.0983} \right) + \left( {1 \times 54.9380} \right) + \left( {4 \times 15.9990} \right) = 158.0323\,{\rm{u}}.\)

Q.3. Is potassium permanganate used to purify water?
Ans: Yes, potassium permanganate is used to purify water. Potassium permanganate is a good oxidizing agent. Therefore, it oxidizes the dissolved iron, manganese, hydrogen sulphide, etc. present in the water into solid particles which can be separated and filtered out easily.

Q.4. Can potassium permanganate kill you?
Ans: Yes, potassium permanganate, if consumed can cause toxic effects such as adult respiratory distress syndrome, pancreatitis, and even death in severe cases.

Q.5. List two uses of potassium permanganate.
Ans: The two uses of potassium permanganate are:
1. It is used as a disinfectant for the purification of water.
2. It is used in the laboratory as Baeyer’s reagent \(\left( {{\rm{KMn}}{{\rm{O}}_4} + {\rm{KOH}}} \right)\) to detect the presence of unsaturation in organic compounds.

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