potassium permanganate

Potassium permanganate
IUPAC name	Potassium manganate(VII)
Other names	Potassium permanganate Potassium manganate(VII) Chameleon mineral Condy's crystals Permanganate of potash
Identifiers
CAS number	7722-64-7 Y
PubChem	24400
EC number	231-760-3
UN number	1490
KEGG	D02053
RTECS number	SD6475000
Properties
Molecular formula	KMnO4
Molar mass	158.034 g/mol
Appearance	purplish-bronze-gray needles magenta–rose in solution
Odor	odorless
Density	2.703 g/cm3
Melting point	240 °C decomp.
Solubility in water	6.38 g/100 ml (20 °C) 25 g/100 mL (65 °C)
Solubility	decomposes in alcohol and organic solvents
Structure
Crystal structure	Orthorhombic
Thermochemistry
Std enthalpy of formation ΔfHo298	−813.4 kJ/mol
Standard molar entropy So298	171.7 J K−1 mol−1
Hazards
MSDS	External MSDS
EU Index	025-002-00-9
EU classification	Oxidant (O) Harmful (Xn) Dangerous for the environment (N)
R-phrases	R8, R22, R50/53
S-phrases	(S2), S60, S61
NFPA 704	3 3 0 OX
Related compounds
Other anions	Potassium manganite Potassium manganate
Other cations	Sodium permanganate Ammonium permanganate
Related compounds	Manganese heptoxide
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Potassium permanganate is the inorganic chemical compound with the formula KMnO₄. It is a salt consisting of K⁺ and MnO−4 ions. Formerly known as permanganate of potash or Condy's crystals, it is a strong oxidizing agent. It dissolves in water to give intense purple solutions, the evaporation of which gives prismatic purplish-black glistening crystals.^[1]

In 2000, worldwide production was estimated at 30 000 tonnes.^[2] In this compound, manganese is in the +7 oxidation state.

Structure

It forms orthorhombic crystals with constants: a = 910.5 pm, b = 572.0 pm, c = 742.5 pm. The Mn-O bond distances are 162.9 pm.

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 + 2 KOH + ½ O₂ → K₂MnO₄ + H₂O

2 MnO2−4 + Cl₂ → 2 MnO−4 + 2 Cl⁻

Permanganates can also be generated by treating a solution of Mn²⁺ ions with strong oxidants such as 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

Almost all applications of potassium permanganate exploit its oxidizing properties.^[2] As a strong oxidant that does not generate toxic byproducts, KMnO₄ has many niche uses.

Potassium permanganate is one of the principle chemicals utilised in the film and television industries to "age" props and set dressings. Its oxidising effects create "hundred year old" or "ancient" looks on hessian cloth, ropes, timber and glass.^[3] It was used on props and sets in films such as "Troy", "300" and "Indiana Jones".^{[citation needed]}

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,^[4][5] and fungal infections of the hands or feet.^[6] 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. Historically it was used to disinfect drinking water.^[7]

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. In histology, it is used 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.^[8][9] Permanganate washes were once used to treat gonorrhea and are still used to treat candidiasis.^[10]

Organic synthesis

Aside from its use in water treatment, the other major application of KMnO₄ is as a reagent for the synthesis of organic compounds.^[11] Significant amounts are required for the synthesis of ascorbic acid, chloramphenicol, saccharine, isonicotinic acid, and pyrazinoic acid.^[2]

Chemistry

Analytical

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. 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 ^[12] is often used.

A solution of KMnO₄ in water, in a volumetric flask

Aqueous, acidic solutions of KMnO₄ are used to collect gaseous mercury in flue gas during stationary source emissions testing.^[13]

Organic

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 decolourises 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 cleaved to give the appropriate carboxylic acid:^[14]

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:^[15]

C₆H₁₃CHO + [O] → C₆H₁₃COOH

Even an alkyl group (with a benzylic hydrogen) on an aromatic ring is oxidized, e.g. toluene to benzoic acid.^[16]

Glycols are highly reactive toward KMnO₄. 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.^[17]

Reaction with acids

Concentrated sulfuric acid reacts with KMnO₄ to give Mn₂O₇, which can be explosive.^[18][19][20] 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) ion (Mn²⁺) and water. 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 reduces 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)

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. 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 can become brown. MnO₂ can be removed by scrubbing with dilute acids.

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.^[21] 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). When solid KMnO₄ is mixed with pure glycerol it will result in a violent combustion reaction.

References

External links

• International Chemical Safety Card 0672
• National Pollutant Inventory: Manganese and compounds Fact Sheet
• The use of potassium permanganate in fish ponds
• Sugar, NaOH and KMnO4