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Potassium permanganate

Potassium permanganate

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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. In 2000, worldwide production was estimated at 30 000 t/a.

It forms orthorhombic crystals with constants: a = 9.105, b = 5.720, c = 7.425 Å. The Mn-O bond distances are 1.629 Å.

Preparation

Potassium permanganate is manufactured on a large scale from manganese dioxide. The mineral pyrolusite is fused with potassium hydroxide and heated in air or with potassium nitrate (a source of oxygen). This process gives potassium manganate, which upon electrolytic oxidation in alkaline solution gives potassium permanganate.
MnO2 + 2OH + O2 → MnO42− + H2O

MnO42− + Cl2 → MnO4 + 2Cl

Permanganates can also be generated by treating a solution of Mn2+ ions with very strong oxidising agents like lead dioxide, PbO2, or sodium bismuthate, NaBiO3. These reactions use the vivid violet colour of permanganate as a test for the presence of manganese.

Uses

In almost all applications of potassium permanganate it is an oxidizing agent. As a strong oxidant that does not generate toxic byproducts, KMnO4 has many niche uses. One of the uses can be said to be as a fixative.

Disinfectant and water treatment

As an oxidant, potassium permanganate can act as a disinfectant. For example, dilute solutions are used to treat canker sores (ulcers), disinfectant for the hands and treatment for mild pompholyx, dermatitis, and fungal infections of the hands or feet. Potassium permanganate, obtainable at pool supply stores, is used in rural areas to remove iron and hydrogen sulfide (rotten egg smell) from well and waste water.

Biomedical uses

Related to the use of KMnO4 for water treatment, this salt is often employed as a specialized disinfectant for treating human and animal ailments. It has been used to induce abortions, although with risks. 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.

Organic synthesis

Aside from its use in water treatment, the other major applicaton of KMnO4 is as a reagent for the synthesis of organic compounds. Significant amounts are required for the synthesis of ascorbic acid, chloramphenicol, saccharine, isonicotinic acid, and pyrazinoic acid.

Analytical chemistry

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. In analytical chemistry, a standardized aqueous solution of KMnO4 is sometimes used as an oxidizing titrant for redox titrations. In a related way, it is used as a reagent to determine the Kappa number of wood pulp. For the standardization of KMnO4 solutions, reduction by oxalic acid is often used.

Aqueous, acidic solutions of KMnO4 are used to collect gaseous mercury in flue gas during stationary source emissions testing.

Organic chemistry

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 oxidizing agent, can react with impurities in a sample.

Under acidic conditions, the alkene double bond is cloven to give the appropriate carboxylic acid:

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

Potassium permanganate oxidizes aldehydes to carboxylic acids, such as the conversion of n-heptanal to heptanoic acid:

C6H13CHO + [O] → C6H13COOH

Even alkyl group (with a benzylic hydrogen) on an aromatic ring are oxidized, e.g. toluene to benzoic acid.

Glycols are highly reactive toward KMnO4. And this vigorous reaction is used in survival kits as a fire starter. For example, a mixture of potassium permanganate and glycerol or pulverized glucose ignites readily. Its sterilizing properties are another reason for inclusion of KMnO4 in a survival kit.

Handling KMnO4

As an oxidizer that generates the dark brown product MnO2, potassium permanganate rapidly stains virtually any organic material such as skin, paper, and clothing. This staining effect is used to "develop" TLC plates. The redox reaction is used for artistic purposes as an agent to prepare paper for fast bleaching. Lemon juice is enough to quickly remove colour from the paper and applied with a paint brush this can create interesting aesthetics. Even glassware containing solutions of KMnO4 become brown. MnO2 can be removed by scrubbing with dilute acids.

Acids and KMnO4

Concentrated sulfuric acid reacts with KMnO4 to give Mn2O7, which can be explosive. 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. 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.
6 KMnO4(aq) + 9 H2SO4(aq) → 6 MnSO4(aq) + 3 K2SO4(aq) + 9 H2O(l) + 5 O3(g)

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. 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 dissolving 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. 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 sometimes still used in torpedoes.

Safety

Solid KMnO4 is a strong oxidizer and thus should be kept separated from oxidizable substances. Reaction with concentrated sulfuric acid produces the highly explosive manganese(VII) oxide (Mn2O7).

References

External links

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