Permanganic acid

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Permanganic acid
Names
IUPAC name
Permanganic acid
Preferred IUPAC name
Permanganic acid
Systematic IUPAC name
Permanganic acid
Other names
Hydrogen permanganate
Hydroxy(trioxo)manganese
Properties
HMnO4
Molar mass 119.94 g mol
Appearance Reddish-violet solution, unstable
Odor Odorless
Melting point Decomposition visible above 40 °C
Boiling point Decomposes
Readily soluble
Solubility Slightly soluble in 1,1,1-trichloro-2,2,2-trifluoroethane, perfluorodecalin
Insoluble in CCl4, chloroform[1]
Hazards
Safety data sheet None
Flash point Non-flammable
Related compounds
Related compounds
Manganese heptoxide
Potassium permanganate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Permanganic acid is an unstable, strong, oxidizing acid with the chemical formula HMnO4.

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.

Extremely concentrated 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 extremely concentrated 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%).

Another route involves the addition of manganese(II) sulfate and lead dioxide (lead(IV oxide)) in sulfuric acid and stirring the suspension:

2 MnSO4 + 5 PbO2 + 3 H2SO4 → 2 HMnO4 + 5 PbSO4 + 2 H2O

The reaction is slow, but it's complete once the lead sulfate stops precipitating. Remove it from the product, either via decantation or filtration. Make sure you perform this temperature at low temperatures, to prevent the permanganic acid from rapidly decomposing. [2]

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. It should be used as soon as it's made. The hydrate can be stored as long as it's frozen (below the freezing point of water).

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.

Rerefences

  1. Frigerio; Journal of the American Chemical Society; vol. 91; nb. 22; (1969); p. 6200 - 6201
  2. http://energetic.proboards.com/thread/47/ammonium-permanganate

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