A few important oxidising reactions of KMnO4 are given below:
1 In acid solutions:
(a) Iodine is liberated from potassium iodide :
10I
– + 2MnO4
– + 16H
+ ®  2Mn
2+ + 8H2O + 5I2
(b) Fe
2+ ion (green) is converted to Fe
3+ (yellow):
5Fe
2+ + MnO4
– + 8H
+ ®  Mn
2+ + 4H2O + 5Fe
3+
Rationalised 2023-24
108
Chemistry
(c) Oxalate ion or oxalic acid is oxidised at 333 K:
5C2O4
2– + 2MnO4
– + 16H
+ ——> 2Mn
2+ + 8H2O + 10CO2
(d) Hydrogen sulphide is oxidised, sulphur being precipitated:
H2S —> 2H
+ + S
2–
5S
2– + 2MnO
–
4 + 16H
+ ——> 2Mn
2+ + 8H2O + 5S
(e) Sulphurous acid or sulphite is oxidised to a sulphate or
sulphuric acid:
5SO3
2– + 2MnO4
– + 6H
+ ——> 2Mn
2+ + 3H2O + 5SO4
2–
(f)
Nitrite is oxidised to nitrate:
5NO2
– + 2MnO4
– + 6H
+ ——> 2Mn
2+ + 5NO3
– + 3H2O
2 In neutral or faintly alkaline solutions:
(a) A notable reaction is the oxidation of iodide to iodate:
2MnO4
– + H2O + I
– ——> 2MnO2 + 2OH
– + IO3
–
(b) Thiosulphate is oxidised almost quantitatively to sulphate:
8MnO4
– + 3S2O3
2– + H2O ——> 8MnO2 + 6SO4
2– + 2OH
–
(c) Manganous salt is oxidised to MnO2; the presence of zinc sulphate
or zinc oxide catalyses the oxidation:
2MnO4
– + 3Mn
2+ + 2H2O ——> 5MnO2 + 4H
+
Note: Permanganate titrations in presence of hydrochloric acid are
unsatisfactory since hydrochloric acid is oxidised to chlorine Uses
Uses
Uses
Uses
Uses: Besides its use in analytical chemistry, potassium permanganate is
used as a favourite oxidant in preparative organic chemistry