Permanganic acid
Permanganic acid is an unstable, strong, oxidizing acid with a formula of HMnO4.
Contents
Properties
Physical
Anhydrous permanganic acid does not exist. Solutions of permanganic acid (up to 20%) are purple liquids, with a smell of copy machines (ozone). The crystallohydrate (HMnO4·2H2O) is a purple crystalline solid that decomposes above 20 degrees Celsius.
Chemical
It is a very strong oxidizer, though milder than its anhydride, manganese heptoxide. It spontaneously decomposes, evolving oxygen and precipitating manganese dioxide, which catalyzes further decomposition of the acid.
By cooling the unstable concentrated solution of the acid near the freezing point, the crystallohydrate can be prepared. Sources vary about the nature of this compound: according to some sources, it is a hydronium salt of permanganic acid, according to others, it is a complex polymanganic acid.
Overconcentrated solutions of this acid can spontaneously decompose with the evolution of ozone. This allows to prepare ozone chemically by reacting potassium permanganate with mid-concentration sulfuric acid (50-80%). This reaction produces an overconcentrated solution of permanganic acid, which promptly decomposes, liberating ozone. Do not use sulfuric acid with a concentration above 80% for this reaction: this may lead to formation of the extremely dangerous anhydride, manganese heptoxide!
Preparation
Permanganic acid can be prepared by dissolving manganese heptoxide in water, or by reacting barium permanganate with sulfuric acid. An impure solution can also be prepared by dissolving potassium permanganate in sulfuric acid of middle concentration (50-80%).
Handling
Safety
Not much is known about the hazards of permanganic acid, but one should treat it with respect, as it is both a strong acid and a strong oxidizer. It may release ozone, which is toxic.
Storage
It is impractical to store this acid because of its instability.
Disposal
Permanganic acid can be dumped in the ground, because it causes the compound to decompose, oxidizing the soil and turning into the safe, insoluble manganese dioxide. But it is better to dispose it after it is already decomposed in the lab.
