Potassium permanganate

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Potassium permanganate
Kmno4.jpg
Different sizes of potassium permanganate crystals.
Names
IUPAC name
Potassium manganate(VII)
Other names
Chameleon mineral
Condy's crystals
Hypermangan
Permanganate of potash
Properties
KMnO4
Molar mass 158.034 g/mol
Appearance Dark purple crystals
Odor Odorless
Density 2.703 g/cm3 (25 °C)
Melting point 240 °C (464 °F; 513 K) (decomposes)
Boiling point Decomposes
6.4 g/100ml (20 °C)
25 g/100 ml (65 °C)
Solubility Reacts with acids, alcohols, glycerol, halogens
Soluble in glacial acetic acid, anhydrous acetonitrile, benzonitrile, pyridine, sulfolane, trifluoroacetic acid
Insoluble in hydrocarbons, halocarbons
Solubility in bromoacetic acid 1.02 g/100 ml
Vapor pressure ~ 0 mmHg
Thermochemistry
171.7 J·mol-1·K-1
−813.4 kJ/mol
Hazards
Safety data sheet Sigma-Aldrich
Flash point Non-flammable
Lethal dose or concentration (LD, LC):
1,090 mg/kg (oral, rat)
Related compounds
Related compounds
Potassium manganate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Potassium permanganate is an ionic compound with the formula KMnO4. At room temperature and pressure it is a deep violet crystalline salt that dissolves to form an intense violet or magenta solution, often tending to black in high concentrations. Potassium permanganate contains manganese in the +7 oxidation state, and is a strong oxidizer commonly encountered in a lab setting. It participates in a wide range of reactions, from simple colour change demonstrations to complex organic syntheses.

Properties

Physical

Potassium permanganate is a dark purple, almost black, crystalline solid that is soluble in water to form an intense magenta solution.
Crystals of potassium permanganate

Chemical

When exposed to light, potassium permanganate will photodecompose to form potassium manganate and manganese dioxide.

Concentrated sulfuric acid dehydrates potassium permanganate to the incredibly unstable manganese heptoxide. Adding a few drops of glycerol (glycerin) to potassium permanganate creates an intense flame hot enough to light thermite and pyrotechnics. Addition of a dilute solution of sodium hydroxide and sucrose to potassium manganate solution and hydrous manganese dioxide creates a "chemical chameleon" reaction, in which the permanganate ion is slowly reduced several times, producing several vivid color changes.

In acidic solutions, permanganates reduce to manganese(II) ions. In neutral and mildly basic solutions, it reduces to manganese dioxide. In extremely basic solutions, the bluish-green manganate ion would form, but it would exist only transiently as it would be reduced to manganese dioxide.

Availability

Like many chemicals, potassium permanganate was easier to find a few years ago, moderately difficult to find today and most likely getting harder and harder to locate in the next few years. It is sometimes sold as a camping chemical because it can light hot fires (when combined with glycerin), decontaminate water and disinfect wounds. Today it is easiest to find potassium permanganate at pharmacies/chemists or in aquarium supply stores as a type of water treatment.

Legality

The sale of potassium permanganate is regulated in several countries, due to its use in cocaine purification and methcathinone production. Check the local laws before using this substance.

Preparation

Making permanganate is a difficult and messy exercise, and although extensive discussion on this topic has been undertaken on Sciencemadness, a method that is accessible to the amateur but also produces worthwhile yields is still yet to be found.

The most popular method involves fusing a mixture of potassium nitrate, potassium hydroxide and manganese dioxide to form crude potassium manganate, then disproportionating it (part of it oxidises the other part, and is reduced as a result) to permanganate and manganese dioxide, with the addition of a dilute solution of sodium bicarbonate (baking soda).

Projects

Handling

Safety

While not overly toxic, potassium permanganate may strongly stain skin or other materials due to the formation of manganese dioxide when reacted with certain organics.

Contact with concentrated sulphuric acid will create a very powerful, unpredictable and unstable oxidizer manganese heptoxide. Accidental creation of this must be avoided, and has been known to create disaster for the backyard chemist.

Potassium permanganate vigorously reacts with many organic compounds, such as glycerol and ethylene glycol, causing some to spontaneously combust. Appropriate care must be taken when combining the two in any setting.

Storage

Potassium permanganate should be stored in closed bottles, away from any acidic vapors or volatile organic compounds. KMnO4 solutions should be covered as they will slowly break down in open air, due to dust or chemical vapors, generally organic.

Disposal

Potassium permanganate can be neutralized with hydrogen peroxide, resulting potassium hydroxide and manganese dioxide. The latter can be recycled through sedimentation or filtration.

References

Relevant Sciencemadness threads