Potassium Permanganate - Structure and Preparation

Structure and Preparation

Potassium permanganate is produced industrially from manganese dioxide, which also occurs as the mineral pyrolusite. The MnO₂ is fused with potassium hydroxide and heated in air or with a source of oxygen, like potassium nitrate or chlorate. This process gives potassium manganate, which upon electrolytic oxidation in alkaline media, or by boiling the manganate solution in the presence of carbon dioxide until all the green color is discharged, gives potassium permanganate.

2 MnO₂ + 4 KOH + O₂ → 2 K₂MnO₄ + 2 H₂O

2 MnO₄2– + Cl₂ → 2 MnO₄– + 2 Cl–

or:

3 K₂MnO₄ + 2 CO₂ → 2 KMnO₄ + 2 K₂CO₃ + MnO₂

In which the potassium permanganate is separated by filtering the insoluble manganese dioxide, evaporating the solution to 1/3 and recrystallizing it.

Permanganate salts can also be generated by treating a solution of Mn2+ ions with strong oxidants such as lead dioxide (PbO₂), or sodium bismuthate (NaBiO₃). Tests for the presence of manganese exploit the vivid violet color of permanganate produced by these reagents.

KMnO₄ forms orthorhombic crystals with constants: a = 910.5 pm, b = 572.0 pm, c = 742.5 pm. The overall motif is similar to that for barium sulfate, with which it forms solid solutions. In the solid (as in solution), each MnO₄- centres are tetrahedral. The Mn-O distances are 1.62 Å.