Potassium permanganate

Uses

Almost all applications of potassium permanganate exploit its oxidizing properties. As a strong oxidant that does not generate toxic byproducts, KMnO₄ has many niche uses.

Infection

As an oxidant, potassium permanganate can act as an antiseptic. 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.

Water treatment

Potassium permanganate is used extensively in the water treatment industry. It is used as a regeneration chemical to remove iron and hydrogen sulfide (rotten egg smell) from well water via a "Manganese Greensand" Filter. "Pot-Perm" is also obtainable at pool supply stores and is used additionally to treat waste water. Historically it was used to disinfect drinking water and can turn the water pink. It currently finds application in the control of nuisance organisms such as zebra mussels in fresh water collection and treatment systems.

Synthesis of organic compounds

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

Analytical use

Potassium permanganate can 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 (permanganometry). 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.

In histology, potassium permanganate was used as a bleaching agent.

Fruit preservation

Ethylene absorbents extend storage time of bananas even at high temperatures. This effect can be exploited by packing bananas in polyethylene together with potassium permanganate. By removing ethylene by oxidation, the permanganate delays the ripening, increasing the fruit's shelf life up to 4 weeks without the need for refrigeration.

Survival kits

Potassium permanganate is typically included in survival kits: as a fire starter (mixed with antifreeze from a car radiator or glycerol), water sterilizer, and for creating distress signals on snow.

Fire service

Potassium permanganate is used in the "plastic sphere dispensers" used to set backfires, burnouts, and prescribed fires. Polymer spheres resembling ping-pong balls containing small amounts of permanganate are injected with ethylene glycol and projected towards the area where ignition is desired, where they spontaneously ignite seconds later. Both handheld and helicopter- or boat-mounted plastic sphere dispensers are used.

Other uses

Potassium permanganate is one of the principal chemicals used in the film and television industries to "age" props and set dressings. Its ready conversion to brown MnO₂ creates "hundred-year-old" or "ancient" looks on Hessian cloth (burlap), ropes, timber, and glass.

Potassium permanganate has been used in attempts to induce abortions at home. These incidents only result in damage to the vagina from the corrosive action of the chemical, since potassium permanganate was proven to be ineffective in producing abortions. The FDA increased legal restrictions on the chemical in response to its use as an abortifacient.

History

In 1659, Johann Rudolf Glauber fused a mixture of the mineral pyrolusite (manganese dioxide, MnO₂) and potassium carbonate to obtain a material that, when dissolved in water, gave a green solution (potassium manganate) which slowly shifted to violet and then finally red. This report represents the first description of the production of potassium permanganate. Just under 200 years later, London chemist Henry Bollmann Condy had an interest in disinfectants; he found that fusing pyrolusite with sodium hydroxide (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 potassium hydroxide (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 to stop competitors from marketing similar products.

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.

Preparation

Potassium permanganate is produced industrially from manganese dioxide, which also occurs as the mineral pyrolusite. The MnO₂ is fused with potassium hydroxide and heated in air or with another source of oxygen, like potassium nitrate or potassium chlorate. This process gives potassium manganate:

2 MnO₂ + 4 KOH + O₂ → 2 K₂MnO₄ + 2 H₂O

(With sodium hydroxide, the end product is not sodium manganate but an Mn(V) compound, which is one reason the potassium permanganate is more commonly used than sodium permanganate. Furthermore, the potassium salt crystallizes better.)

The potassium manganate is then converted into permanganate by electrolytic oxidation in alkaline media:

2 K₂MnO₄ + 2 H₂O → 2 KMnO₄ + 2 KOH + H₂

Other methods

Although of no commercial importance, potassium manganate can be oxidized by chlorine or by disproportionation under acid conditions. The chlorine oxidation reaction is

2 K₂MnO₄ + Cl₂ → 2 KMnO₄ + 2 KCl

And the acid-induced disproportionation reaction may be written as

3 K₂MnO₄ + 4 HCl → 2 KMnO₄ + MnO₂ + 2 H₂O + 4 KCl

A weak acid such as carbonic acid is sufficient for this reaction:

3 K₂MnO₄ + 2 CO₂ → 2 KMnO₄ + 2 K₂CO₃ + MnO₂

Potassium permanganate crystallizes from the concentrated reaction solution.

Permanganate salts can also be generated by treating a solution of Mn²⁺ ions with strong oxidants such as lead dioxide (PbO₂), sodium bismuthate (NaBiO₃), or peroxydisulphate. Tests for the presence of manganese exploit the vivid violet color of permanganate produced by these reagents.

Structure

KMnO₄ forms orthorhombic crystals with constants: a = 910.5 pm, b = 572.0 pm, c = 742.5 pm. The overall motif is similar to that for barium sulfate, with which it forms solid solutions. In the solid (as in solution), each MnO₄⁻ centres are tetrahedral. The Mn–O distances are 1.62 Å.

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 initially purple permanganate solution and generates a brown precipitate (MnO₂). 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 a variety of groups.

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

CH₃(CH₂)₁₇CH=CH₂ + 2 KMnO₄ + 3 H₂SO₄ → CH₃(CH₂)₁₇COOH + CO₂ + 4 H₂O + K₂SO₄ + 2 MnSO₄

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

5 C₆H₁₃CHO + 2 KMnO₄ + 3 H₂SO₄ → 5 C₆H₁₃COOH + 3 H₂O + K₂SO₄ + 2 MnSO₄

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

5 C₆H₅CH₃ + 6 KMnO₄ + 9 H₂SO₄ → 5 C₆H₅COOH + 14 H₂O + 3 K₂SO₄ + 6 MnSO₄

Glycols and polyols are highly reactive toward KMnO₄. For example, addition of potassium permanganate to an aqueous solution of sugar and sodium hydroxide produces the chemical chameleon reaction, which involves dramatic color changes associated with the various oxidation states of manganese. A related vigorous reaction is exploited as a fire starter in survival kits. 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.

By itself, potassium permanganate does not dissolve in many organic solvents. If an organic solution of permanganate is desired, "purple benzene" may be prepared, either by treating a two phase mixture of aqueous potassium permanganate and benzene with a quaternary ammonium salt, or by sequestering the potassium cation with a crown ether.

Reaction with acids

Concentrated sulfuric acid reacts with KMnO₄ to give Mn₂O₇, which can be explosive. Its reaction with 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) ion (Mn²⁺) and water. In neutral solution, permanganate is only reduced by three electrons 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 reduces in an alkaline solution to green K₂MnO₄, wherein manganese is in the +6 oxidation state.

A curious reaction occurs upon addition of 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 the reaction also produces explosive Mn₂O₇, this should only be attempted with great care.

Thermal decomposition

Solid potassium permanganate decomposes when heated:

2KMnO₄ → K₂MnO₄ + MnO₂(s) + O₂

Safety and handling

As an oxidizer that generates the dark brown product MnO₂, potassium permanganate rapidly stains virtually any organic material such as skin, paper, and clothing. Solid KMnO₄ is a strong oxidizer, thus should be kept separated from oxidizable substances such as skin, paper, and clothing. Reaction with concentrated sulfuric acid produces the highly explosive manganese(VII) oxide (Mn₂O₇). When solid, KMnO₄ mixed with pure glycerol or other simple alcohols will result in a violent combustion reaction.

3 C₃H₅(OH)₃ + 14 KMnO₄ → 14 MnO₂ + 7 K₂CO₃ + 2 CO₂ + 12 H₂O