Manganese(VII) oxide is obtained as an oil with a green metallic lustre in reflected light by the action of concentrated sulphuric acid on very pure potassium permanganate. The compound is extremely explosive and great care must be observed in the preparation and handling of this compound. The compound dissociates at about 55°C and detonates at 95°C. In a vacuum rapid and explosive decomposition occurs above 10°C. During explosive decomposition α-Mn2O3 results, but when dissociation is slow γ-MnO2 is formed. MnO7 is hygroscopic and in moist air it decomposes to Mn02 and a mixture of ozone and oxygen. The compound is stable below -10°C in anhydrous conditions. It reacts explosively with most organic compounds and attacks carbon tetrachloride even below room temperature.
Concentrated H2SO4 (15 ml., d 1.84) is placed in a dry porcelain mortar precleaned with chromo sulfuric acid; then 23 g of KMnO4 is carefully added over 10-15 minutes while constantly stirring with a pestle. The following precautions must be observed to obtain the desired result: only very pure KMnO4 crystals, free of dust and organic substances (do not reduce the crystals to powder), can be used. The reaction slurry should be left standing overnight in a dry spot, protected against dust. Porous pyrolusite is formed during this time, and the Mn2O7 oil is very gently kneaded out from it. Proper safety measures must be observed during the preparation and further workup of the material since it often explodes for no apparent reason (an asbestos face shield with safety glasses and heavy leather gloves should be worn, and one should work behind a heavy glass plate and a fine wire screen). Yield: 10 g. (62%). The product is entirely free of K+ and SO42-.
2 KMnO4 + 2 H2SO4 = 2 KHSO, + Mn2O7 + H2O
