Potassium permanganate - KMnO4
Potassium permanganate is prepared from pyrolusite (MnO2) ore. The preparation involves the following steps.
i) Conversion of MnO2 to potassium manganate:
Powdered ore is fused with KOH in the presence of air or oxidising agents like KNO3 or KClO3. A green coloured potassium manganate is formed.
2MnO2 + 2KOH + O2 → K2MnO4 + 2H2O
K2MnO4 = potassium manganate (Green)
ii) Oxidation of potassium manganate to potassium permanganate: Potassium manganate thus obtained can be oxidised in two ways , either by chemical oxidation or electrolytic oxidation.
In this method potassium manganate is treated with ozone (O3) or chlorine to get potassium permanganate.
2MnO42− + O3 + H2O → 2MnO4− + 2OH− + O2
2MnO42− + Cl2 → 2MnO4− + 2Cl−
In this method aqueous solution of potassium manganate is electrolyzed in the presence of little alkali.
K2MnO4 ↔ 2K+ + MnO42−
H2O ↔ H+ + OH−
Manganate ions are converted into permanganate ions at anode.
MnO42− ↔ MnO4− + e−
H2is liberated at the cathode.
2H+ + 2e− → H2 ↑
The purple coloured solution is concentrated by evaporation and forms crystals of potassium permanganate on cooling.
Potassium permanganate exists in the form of dark purple crystals which melts at 513 K.
It is sparingly soluble in cold water but, fairly soluble in hot water.
Permanganate ion has tetrahedral geometry in which the central Mn7+ is sp3 hybridised.
When heated, potassium permanganate decomposes to form potassium manganate and manganese dioxide.
2KMnO4 → 2K2MnO4 + MnO2 + O2
On treating with cold conc H2SO4, it decomposes to form manganese heptoxide, which subsequently decomposes explosively.
2KMnO4 + 2H2SO4 (cold) → Mn2O7 + 2KHSO4 + H2O (cold)
Mn2O7 →Δ→ 2MnO2 + 3O2
But with hot conc H2SO4, potassium permanganate give MnSO4
4KMnO4 + 6H2SO4(hot) → 4MnSO4 + 2K2SO4 + 6H2O + 5O2 (hot)
Potassium permanganate is a strong oxidising agent, its oxidising action differs in different reaction medium.
In neutral medium, it is reduced to MnO2.
MnO4− + 2H2O + 3e− → MnO2 + 4OH−
(i) It oxidises H2S to sulphur
2MnO4− + 3H2S → 2MnO2 + 3S + 2OH− + 2H2O
(ii) It oxidises thiosulphate into sulphate
8MnO4− + 3S2O32− + H2O → 6SO42− + 8MnO2 + 2OH−
In the presence of alkali metal hydroxides, the permanganate ion is converted into manganate.
MnO4− + e− → MnO42−
This manganate is further reduced to MnO2 by some reducing agents.
MnO42− + H2O → MnO2 + 2OH− + [O]
So the overall reaction can be written as follows.
MnO4− + 2H2O + 3e− → MnO2 + 4OH−
This reaction is similar as that for neutral medium.
Cold dilute alkaline KMnO4 is known as Bayer’s reagent. It is used to oxidise alkenes into diols. For example, ethylene can be converted into ethylene glycol and this reaction is used as a test for unsaturation.
In the presence of dilute sulphuric acid, potassium permanganate acts as a very strong oxidising agent. Permanganate ions is converted into Mn2+ ion.
MnO4− + 8H+ + 5e− → Mn2+ + 4H2O
The oxidising nature of potassium permanganate (permanganate ion) in acid medium is illustrated in the following examples.
(i) It oxidises ferrous salts to ferric salts.
2MnO4− + 10Fe2+ + 16H+ → 2Mn2+ + 10Fe3+ + 8H2O
(ii) It oxidises iodide ions to iodine
2MnO4− + 10I− + 16H+ → 2Mn2+ + 5I2 + 8H2O
(iii) It oxidises oxalic acid to CO2
2MnO4− + 5( COO)2− + 16H+ → 2Mn2+ + 10CO2 + 8H2O
(iv) It oxidises sulphide ion to sulphur
2MnO4− + 5 S2− + 16H+ → 2Mn2+ + 5 S + 8H2O
(vi) It oxidises nitrites to nitrates
2MnO4− + 5NO2− + 6H+ → 2Mn2+ + 5NO3− + 3H2O
(vi) It oxidises alcohols to aldehydes.
2KMnO4 + 3H2SO4 + 5CH3CH2OH → 2K2SO4 + 2MnSO4 + 5CH3CHO + 8H2O
(vii) It oxidises sulphite to sulphate
2MnO4− + 5SO32− + 6H+ → 2Mn2+ + 5SO42− + 3H2O
Some important uses of potassium permanganate are listed below.
1. It is used as a strong oxidizing agent.
2. It is used for the treatment of various skin infections and fungal infections of the foot.
3. It used in water treatment industries to remove iron and hydrogen sulphide from well water.
4. It is used as a Bayer’s reagent for detecting unsaturation in an organic compound.
5. It is used in quantitative analysis for the estimation of ferrous salts, oxalates, hydrogen peroxide and iodides.
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