Potassium permanganate

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." The permanganate ion is a strong oxidizing agent. It dissolves in water to give deep purple solutions, evaporation of which gives prismatic purplish-black glistening crystals.^[1] It has a sweet flavour.

Its crystal structure is orthorhombic with constants: a = 9.105, b = 5.720, c = 7.425 Å. Distance between Mn and O atoms in the tetrahedral anions is 1.629 Å.

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Preparation

Potassium permanganate is manufactured on a large scale due to its manifold uses in the laboratory. In the first stage, pyrolusite, which is manganese dioxide in its natural form, is fused with potassium hydroxide and heated in air or with potassium nitrate (a source of oxygen). This leads to the formation of potassium manganate, which on electrolyic oxidation in alkaline solution gives potassium permanganate.

MnO₂ + 2OH^- + O₂ → MnO₄^2- + H₂O

MnO₄^2- + Cl₂ → MnO₄^- + 2Cl^-

Permanganates can also be prepared by treating a solution of Mn²⁺ ions with very strong oxidising agents like lead dioxide, PbO₂, or sodium bismuthate, NaBiO₃, and these reactions have been used to test for the presence of manganese due to the formation of the distinctly violet color of permanganate.

Uses

Almost all applications of potassium permanganate are derived from it being an oxidizing agent in diverse chemical reactions in the laboratory and in industry.^[2]

Disinfectant and water treatment

As an oxidant, potassium permanganate can act as a disinfectant. For example, dilute solutions are used as a treatment for canker sores (ulcers), disinfectant for the hands and treatment for mild pompholyx, dermatitis, and fungal infections of the hands or feet.^{[citation needed]}

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 by oxidation. Some degree of disinfection may also be achieved from this use. Potassium permanganate similarly removes hydrogen sulfide from waste water.

KMnO₄ 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 (a source of glycols) to produce flame, although this can be dangerous and should be done in a controlled manner, such as dipping some paper into the anti-freeze and then adding a small amount of potassium permanganate. The ability to sterilise water and wounds is another reason for inclusion in a survival kit.

Potassium permanganate can also be used to quantitatively determine the total oxidisable organic material in an aqueous sample. The value determined is known as the permanganate value.

Biomedical uses

Dilute solution introduced into the uterus have been used to self-induce abortions. Insertion of the crystalline form as an abortifacient into the vagina has also been attempted, often resulting in severe injury.^[3]

A dilute solution of acidified potassium permanganate is used in histology to bleach melanin which obscures tissue detail. Potassium permanganate can also be used to differentiate amyloid AA from other types of amyloid pathologically deposited in body tissues. Incubation of fixed tissue with potassium permanganate will prevent amyloid AA from staining with congo red whereas other types of amyloid are unaffected.^[4][5]

Miscellaneous uses

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.

• A dilute solution (10mg/l) of potassium permanganate can be used to eliminate snails from plants prior to placing them in a fresh-water aquarium.^{[citation needed]}

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 a related way, it is used as a reagent to determine the Kappa number of wood pulp.

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 as a disinfectant for vegetables such as lettuce.

In biochemistry, permanganate footprinting is a technique that is applied.

Reactions

Organic synthesis

Dilute solutions of KMnO₄ convert alkenes into diols (glycols). This behaviour is also used as a qualitative test for the presence of double or triple bonds in a molecule, since the reaction decolorizes the permanganate solution; thus it is sometimes referred to as Baeyer's reagent. However, bromine serves better in measuring unsaturation (double or triple bonds) quantitatively, since KMnO₄, being a very strong oxidising agent can react with impurities in a sample.

Under acidic conditions, the alkene double bond is cleaved to give the appropriate carboxylic acid:^[6]

CH₃(CH₂)₁₇CH=CH₂ + [O] → CH₃(CH₂)₁₇COOH

Potassium permanganate oxidizes aldehydes to carboxylic acids, such as with n-heptanal:^[7]

C₆H₁₃CHO + [O] → C₆H₁₃COOH

Concentrated solutions oxidize a methyl group (or any other alkyl group with a benzylic hydrogen) on an aromatic ring, e.g. toluene to benzoic acid.

KMnO₄ oxidizes 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 II 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.^[8]

KMnO₄ is also used in removal of impurities from coca base in the production of cocaine. The purer base results in aesthetically pleasing white crystals.^[9]

Acids and KMnO₄

Concentrated sulfuric acid reacts with KMnO₄ to give Mn₂O₇, which can be explosive.^[10][11][12]Similarly concentrated hydrochloric acid gives chlorine. The Mn-containing products from redox reactions depend on the pH. Acidic solutions of permanganate are reduced to the faintly pink manganese(II) sulfate ([Mn(H₂O)₆]²⁺). In neutral solution, permanganate is only reduced by 3e⁻ to give MnO₂, wherein Mn is in a +4 oxidation state. This is the material that stains one's skin when handling KMnO₄. KMnO₄ spontaneously reduced in an alkaline solution to green K₂MnO₄, wherein manganese is in the +6 oxidation state.

A curious reaction is produced by adding concentrated sulfuric acid to potassium permanganate. Although no reaction may be apparent, the vapor over the mixture will ignite paper impregnated with alcohol. 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. As a similar reaction produces explosive Mn₂O₇, this should only be attempted with great care. An approximate equation for the ozone formation is shown below.

At room temperature

6 KMnO₄(aq) + 9 H₂SO₄(aq) → 6 MnSO₄(aq) + 3 K₂SO₄(aq) + 9 H₂O(l) + 5 O₃(v)

History

In 1659 a German chemist, J.R. Glauber, fused a mixture of the mineral pyrolusite and potassium carbonate to obtain a material that, when dissolved in water, gave a green solution (potassium manganate) which slowly shifted to violet potassium permanganate and then finally red. This report represents the first description of the production of potassium permanganate.^[13]

Just under two hundred years later London chemist Henry Bollmann Condy had an interest in disinfectants, and marketed several products including ozonised water. He found that fusing pyrolusite with NaOH and dissolved it in water produced a solution with disinfectant properties. He patented this solution, and marketed it as Condy's Fluid. Although effective, the solution was not very stable. This was overcome by using KOH rather than NaOH. This was more stable, and had the advantage of easy conversion to the equally effective potassium permanganate crystals. This crystalline material was known as Condy’s crystals or Condy’s powder. Potassium permanganate was comparatively easy to manufacture so Condy was subsequently forced to spend considerable time in litigation in order to stop competitors from marketing products similar to Condy's Fluid or Condy's Crystals.

Early photographers used it as a component of flash powder. It is now replaced with other oxidizers, due to the instability of permanganate mixtures.

Safety

Solid KMnO₄ is a strong oxidizer and in general it should be kept separated from oxidizable substances. Dilute aqueous solutions of KMnO₄ are not dangerous. KMnO₄ forms dangerous products upon contact with concentrated acids. For instance, a reaction with concentrated sulfuric acid produces the highly explosive manganese(VII) oxide (Mn₂O₇).

As an oxidizer, potassium permanganate stains the hand and clothing as it is reduced to MnO₂. Clothing stains may be washed away using acetic acid. Skin stains, which are typically brown, disappear within 48 hours.

References