Describe the preparation of potassium permanganate. How does the acidified permanganate solution react with (i) iron(II) ions (ii) $S O_{2}$ and (iii) oxalic acid? Write the ionic equations for the reactions.

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Solution

Preparation of potassium permanganate, $K M n O_{4}$ from pyrolusite ore $M n O_{2}$ .
(i) Pyrolusite ore is fused with $K O H$ (or $K_{2} C O_{3}$ ) in presence of air (or oxidising agents such as $K N O_{3}$ or $K C l O_{3}$ ) to give potassium manganate $K_{2} M n O_{4}$ (green mass).
$2 M N O_{2} + 4 K O H + O_{2} D e l t a - -- \to 2 K_{2} M n O_{4} + 2 H_{2} O$
(ii) Extraction of green mass with water followed by oxidation with chlorine (or ozone or $C O_{2}$ ) gives potassium permanganate.
$2 K_{2} M n O_{4} + C l_{2} \to 2 K C l + 2 K M n O_{4}$
On concentrating the solution, dark purple crystals of potassium permanganate separates out.
Note: In neutral or acidic medium, manganate ion will disproportionate to permanganate ion.
$3 M n O_{4}^{2 -} + 4 H^{+} \to 2 M n O_{4}^{-} + M n O_{2} + 2 H_{2} O$
Convertion of manganate to permanganate can also be carried by electrolytic oxidation.
Reactions of acidified permanganate solution with (i) iron(II) ions (ii) $S O_{2}$ and (iii) oxalic acid:
$2 M n O_{4}^{-} + 10 F e^{2 +} + 16 H^{+} \to 2 M n^{2 +} + 10 F e^{3 +} + 8 H_{2} O$
$2 M n O_{4}^{-} + 5 S O_{2} + 2 H_{2} O \to 5 S O_{4}^{2 -} + 2 M n^{2 +} + 4 H^{+}$
$2 M n O_{4}^{-} + 5 C_{2} O_{4}^{2 -} + 16 H^{+} \to 2 M n^{2 +} + 8 H_{2} O + 10 C O_{2}$

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(ii) $S o_{2}$
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Write the ionic equations for the reactions.

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(i) iron (ii) ions (ii) $S O_{2}$ ?

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