Question

Describe the preparation of potassium permanganate. How does the acidified permanganate solution react with SO₂? Write the ionic equation for the reaction.

Indicate the steps in the preparation of KMnO₄ from pyrolusite ore.

Answer 1

Potassium permanganate can be made by the following methods –

It is produced industrially by the electrolytic oxidation of manganate (VI), followed by the alkaline oxidative fusion of MnO₂.

$$\mathrm{MnO}{\phantom{A}}_2\stackrel{\mathrm{Fused}\mathrm{with}\mathrm{KOH},\mathrm{oxidised}\mathrm{with}\mathrm{air}\mathrm{or}\mathrm{KNO}{\phantom{A}}_3}{\to }\underset{\mathrm{manganate}\mathrm{ion}}{\mathrm{MnO}{\phantom{A}}_4^{2-}}$$

$$\underset{\mathrm{manganate}}{\mathrm{MnO}{\phantom{A}}_4^{2-}}\stackrel{\mathrm{Electrolytic}\mathrm{oxidation}\mathrm{in}\mathrm{alkaline}\mathrm{solution}}{\to }\underset{\mathrm{permanganate}\mathrm{ion}}{\mathrm{MnO}{\phantom{A}}_4^{-}}$$

The chemical reactions of acidic potassium permanganate are as follows –

$$2\mathrm{MnO}{\phantom{A}}_4^{-}+2\mathrm{H}{\phantom{A}}_2\mathrm{O}+5\mathrm{SO}{\phantom{A}}_2⟶2\mathrm{Mn}{\phantom{A}}^{2+}+4\mathrm{H}{\phantom{A}}^{+}+5\mathrm{SO}{\phantom{A}}_4^{2-}$$

Answer 2

$$2\mathrm{MnO}{\phantom{A}}_2+4\mathrm{KOH}+\mathrm{O}{\phantom{A}}_2\stackrel{\mathrm{\Delta }}{\to }2\mathrm{K}{\phantom{A}}_2\mathrm{MnO}{\phantom{A}}_4+2\mathrm{H}{\phantom{A}}_2\mathrm{O}$$

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