Chemistry · Ch 4 — Transition and Inner Transition Elements
Preparation of Potassium Permanganate
Preparation of Potassium Permanganate
The industrial preparation of potassium permanganate proceeds from pyrolusite ore (chemically manganese dioxide, MnO₂) in two clearly separated stages. In the first stage, powdered pyrolusite ore is fused with solid potassium hydroxide (KOH) in the presence of atmospheric air, or with an added oxidising agent such as potassium nitrate (KNO₃) or potassium chlorate (KClO₃) to accelerate and ensure the oxidation. This fusion step is described by the equation 2MnO₂ + 4KOH + O₂ → 2K₂MnO₄ + 2H₂O, and it lifts the manganese from the +4 oxidation state present in MnO₂ up to the +6 oxidation state present in potassium manganate, K₂MnO₄ -- a compound readily recognisable by its deep green colour.
In the second stage, this green potassium manganate intermediate must be oxidised one further step, from the +6 state up to the +7 state, to give the final purple potassium permanganate product. The textbook describes two distinct routes by which this second-stage oxidation can be carried out: chemical oxidation and electrolytic oxidation.
In the chemical-oxidation route, potassium manganate in aqueous solution is treated directly with a suitable chemical oxidant -- either ozone (O₃) or chlorine gas (Cl₂) -- to complete the oxidation to permanganate. With ozone: 2MnO₄²⁻ + O₃ + H₂O → 2MnO₄⁻ + 2OH⁻ + O₂. With chlorine: 2MnO₄²⁻ + Cl₂ → 2MnO₄⁻ + 2Cl⁻. Either oxidant removes one electron per manganate ion, converting the green Mn(VI) species into the purple Mn(VII) permanganate ion. …
Worked out. 2MnO₂ + 4KOH + O₂ → 2K₂MnO₄ + 2H₂O -- powdered pyrolusite ore is fused with solid KOH in the presence of air or an added oxidising agent such as KNO₃ or KClO₃, converting the manganese from the +4 state in MnO₂ to the +6 state in potassium manganate, K₂MnO₄, which is recognisable by its deep green colour and is the intermediate en route to potassium permanga …
Worked out. By ozone: 2MnO₄²⁻ + O₃ + H₂O → 2MnO₄⁻ + 2OH⁻ + O₂. By chlorine: 2MnO₄²⁻ + Cl₂ → 2MnO₄⁻ + 2Cl⁻. Either oxidant lifts manganese from the +6 state (green manganate) to the +7 state (purple permanganate) by removing one electron per manganate ion, offering an alternative to the electrolytic anodic-oxidation route described next. …
Worked out. An aqueous solution of potassium manganate, in the presence of a little alkali, is electrolysed: at the anode, manganate ions are oxidised to permanganate ions, MnO₄²⁻ → MnO₄⁻ + e⁻ (green to purple); at the cathode, water is reduced and hydrogen gas is liberated, 2H⁺ + 2e⁻ → H₂↑. The purple solution is then concentrated by evaporation and cooled to crystallise potassium permanganate …