Manufacture Of Kmno4 — the NEET Chemistry reaction: mechanism, reagents, conditions, structures and exam traps.
Preparation of Potassium Permanganate Potassium permanganate is manufactured industrially from pyrolusite (MnO₂) via oxidative alkaline fusion with KOH and air to produce potassium manganate (K₂MnO₄), followed by conversion of manganate to permanganate. The conversion step is accomplished either by electrolytic oxidation in alkaline solution or by chemical routes such as chlorine oxidation or controlled disproportionation. Black-brown MnO₂ turns into a green melt/solution of K₂MnO₄ on fusion and leaching. During electrolysis, purple color of KMnO₄ develops at the anode compartment and H₂ gas evolves at the cathode. Disproportionation with CO₂ produces a brown MnO₂ precipitate and intensifying purple KMnO₄ solution; chlorine oxidation gives rapid purple coloration with the characteristic pungent smell of Cl₂. The oxidative alkaline fusion of MnO₂ to K₂MnO₄ is exothermic (ΔH < 0), whereas the conversion of manganate to permanganate by electrolysis is non-spontaneous and requires electrical work. Manganate is stabilized in strongly alkaline medium; lowering [OH⁻] shifts equilibrium toward disproportionation to permanganate and MnO₂. 1. Oxidative alkaline fusion: Finely ground pyrolusite (MnO₂) is fused with solid KOH under a stream of air (or with an added oxidant such as KClO₃/KNO₃) to form potassium manganate: 2 , MnO 2 (s) + 4 , KOH (s) + O 2 (g) 523 – 573 , K 2 , K 2 MnO 4 (s) + 2 , H 2 O (g). 2. Leaching and clarification: The green melt containing K₂MnO₄ is leached with hot water to dissolve manganate; insoluble gangue and unreacted solids are removed by filtration, yielding a clear green manganate solution. 3. Electrolytic route (industrial): The alkaline K₂MnO₄ solution is electrolyzed in a cell (Ni or stainless-steel anode; Fe or steel cathode). Anodic oxidation: MnO 4 2- MnO 4 - + e - . Cathodic reduction: H 2 O + e - 1 2 , H 2 (g) + OH - . Net (with K⁺): K 2 MnO 4 (aq) + H 2 O (l) [ Ni anode ] electrolysis KMnO 4 (aq) + 1 2 , H 2 (g) + KOH (aq). 4. Alternative chemical oxidation: The green manganate solution can be oxidized with chlorine gas to permanganate without changing the medium significantly: 2 , K 2 MnO 4 (aq) + Cl 2 (g) 2 , KMnO 4 (aq) + 2 , KCl (aq). 5. Disproportionation route (on lowering alkalinity): If the alkalinity of manganate solution is decreased (e.g., by passing CO₂), manganate disproportionates: 3 , K 2 MnO 4 (aq) + 2 , H 2 O (l) CO 2 2 , KMnO 4 (aq) + MnO 2 (s) + 4 , KOH (aq). The MnO₂ precipitate is filtered off. 6. Concentration and crystallization: The purple KMnO₄ solution is concentrated under controlled conditions and cooled to crystallize dark purple KMnO₄, which is separated and dried. Forgetting the sequence Mn(+4) → Mn(+6) (manganate, green) → Mn(+7) (permanganate, purple). Assuming simple acidification of manganate gives only KMnO₄; actually it disproportionates to KMnO₄ + MnO₂ in less alkaline/near-neutral medium. Mixing up oxidants: air/KClO₃/KNO₃ are used in the fusion step; Cl₂ is used to oxidize manganate to permanganate (not the other way around). Neglecting that H₂ gas is evolved at the cathode during electrolytic oxidation. Quoting a catalyst (e.g., V₂O₅) — none is used; it is oxidative fusion followed by electrolysis or chemical oxidation.