Potassium permanganate

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Potassium permanganate
General
Other names	Potassium manganate(VII)
Molecular formula	KMnO₄
Molar mass	158.04 g/mol
Appearance	purple solid
CAS number	^*
Properties
Density and phase	2.703 g/cm³, solid
Solubility in water	6.38 g/100 ml (20 °C)
Melting point	270 °C decomp.
Thermodynamic data
Standard enthalpy of	−813.4 kJ/mol
Standard molar entropy S°_solid	171.7 J.K⁻¹.mol⁻¹
Hazards
EU classification	Oxidant (O) Harmful (Xn) Dangerous for the environment (N)
NFPA 704
R-phrases	R8, R22, R50/53
S-phrases	S2, S60, S61
Supplementary data page
Structure and properties	n, ε_r, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Regulatory data	Flash point, RTECS number, etc.
Related compounds
Other anions	Potassium perrhenate
Other cations	Sodium permanganate
Related compounds	Potassium manganite Potassium manganate Manganese heptoxide
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Chemical infobox

Potassium permanganate is the chemical compound KMnO₄. In this salt, manganese is in the +7 oxidation state. The salt is also known as "permanganate of potash" and "Condy's crystals". The permanganate ion is a strong oxidizing agent. It dissolves in water to give deep purple solutions, evaporation of which gives prismatic purple-black glistening crystals. It has a sweet taste and is odorless.

History

Potassium permanganate was discovered in 1659.

Early photographers used it as a component of flash powder.

Chemical applications

Potassium permanganate is used as an oxidizing agent in diverse chemical reactions in the laboratory and in industry. It also serves as a disinfectant and in deodorizers. It can be used as a reagent for the synthesis of many different kinds of chemical compounds. For example, a dilute solution of KMnO₄ can convert an organic compound with a carbon-carbon double bond into a diol (glycol) compound. Stronger permanganate solutions can oxidize a methyl group on an aromatic ring to a carboxyl group. In analytical chemistry, a standardized aqueous solution of KMnO₄ is sometimes used as an oxidizing titrant for redox titrations due to its deep purple color.

In acidic solutions, permanganate is reduced to the faintly pink ^*⁺². In neutral solution, permanganate is only reduced by 3e^- to give a brown precipitate of MnO₂, wherein Mn is in a +4 oxidation state. KMnO₄ spontaneously reduced in an alkaline solution to K₂MnO₄, wherein manganese is in the +6 oxidation state.

KMnO₄ can be used in an oxidative reaction with pseudoephedrine hydrochloride to produce methcathinone, a Schedule I drug in the United States. Consequently the DEA has restricted its use and sale by classifying it as a List I controlled precursor. Potassium permanganate is listed as a Table I precursor under the United Nations Convention Against Illicit Traffic in Narcotic Drugs and Psychotropic Substances(Red list).

Aqueous solutions of KMnO₄ have been used together with T-Stoff (i.e. 80 % hydrogen peroxide) as propellant for the rocket plane Messerschmitt Me 163. In this application, it was known as Z-Stoff. This combination of propellants is still used in torpedoes.

It is used as a reagent to determine the Kappa number of wood pulp.

Biomedical uses

KMnO₄ is employed to treat some parasitic diseases of fish, in treatment of drinking water, as well as an antidote in phosphorus poisoning. In Africa, it has been used to as a disinfectant for vegetables such as lettuce.

Dilute solutions are used as a mouthwash (0.25%), disinfectant for the hands (about 1%) and treatment for mild pompholyx dermatitis or fungal infections of the hands or feet.

A dilute (10mg/l) of potassium permanganate can be used to eliminate snails from plants prior to placing them in a fresh-water aquarium.

The intriguing story of ozone and permanganate

A curious reaction is observed when one adds concentrated sulfuric acid to potassium permanganate. Although no reaction may be apparent, the vapor over the mixture will ignite paper impregnated with alcohol. In principle (although not recommended!), this reaction could be used to produce rapid fire in a laboratory, requiring no matches or other ignition sources. Potassium permanganate and sulfuric acid react to produce some ozone, which has a high oxidising power and rapidly oxidises the alcohol, causing it to combust. An approximate equation for the ozone formation is shown below.

6 KMnO₄ + 9 H₂SO₄ → 6 MnSO₄ + 3 K₂SO₄ + 9 H₂O + 5 O₃

The H₂SO₄/KMnO₄ reaction can also produce the oily Mn₂O₇, which is unstable and can decompose explosively.

Cautions

Solid KMnO₄ is a strong oxidizer and in general, it should be kept separated from oxidisible substances. When mixed with pure glycerine, it causes a highly exothermic chemical reaction. Thermite is sometimes ignited using this reaction. This reaction would turn red hot as a spontaneous "combustion" which would melt a glass or other container holding the reacting contents and could ignite anything flammable nearby. A comparable reaction could occur when solid KMnO₄ is mixed with other organic materials. Aqueous solutions of KMnO₄ are much less dangerous, especially when dilute. Solid KMnO₄ with concentrated sulfuric acid forms Mn₂O₇, which can be explosive. For example, relatively pure powdered potassium permanganate + powdered sugar will ignite (but not explode) a few seconds after a drop of water is added.

Potassium permanganate stains the hand and clothing (as it is reduced to MnO₂) and should be handled with care. Clothing stains may be washed away using acetic acid. Skin stains disappear within 48 hours. In addition, mixing solid KMnO₄ with concentrated hydrochloric acid generates dangerous chlorine gas. ^*

Common Usage

Potassium permanganate is often included in survival kits along with either glycerine or a glucose tablet for the purposes of making fire. The glucose tablet can be ground up, mixed with the potassium permanganate and caused to combust by applying friction. It can also be mixed with anti-freeze from a vehicle to produce flame, although this can be dangerous and should be done in a controlled manner ie dipping some paper into the anti-freeze and then adding a small amount of potassium permanganate. The ability to sterilise water and wounds is also advantageous and another reason for inclusion in a survival kit.

High-grade potassium permanganate can be found at pool supply stores and is used in rural areas to remove iron and hydrogen sulfide (rotten egg smell) from well water.

References

Fatiadi, A. J. Synthesis 1987, 85-127. (Review) ()
F. A. Cotton, G. Wilkinson, C. A. Murillo, and M. Bochmann (April 1999). Advanced Inorganic Chemistry, 6th Edition. Wiley-VCH. ISBN 0-471-19957-5

Dzhabiev, T. S.; Denisov, N. N.; Moiseev, D. N. and Shilov, A. E., "Formation of Ozone During the Reduction of Potassium Permanganate in Sulfuric Acid Solutions", Russian Journal of Physical Chemistry, 2005, volume 79, pages 1755-1760.
Barthel, H. and Duvinage, B., "Clemens Winkler. His Experiments with Ozone in 1892", Praxis der Naturwissenschaften, Chemie, 2000, volume 49, page 18ff.

External links

Potassium compounds | Permanganates | Reagents for organic chemistry

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