| Potassium permanganate | |
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Potassium manganate(VII) |
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Other names
Potassium permanganate |
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| Identifiers | |
| CAS number | 7722-64-7 |
| PubChem | 24400 |
| ChemSpider | 22810 |
| EC number | 231-760-3 |
| UN number | 1490 |
| KEGG | D02053 |
| RTECS number | SD6475000 |
| Jmol-3D images | Image 1 |
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| 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, 513 K, 464 °F (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 ΔfH |
−813.4 kJ/mol |
| Standard molar entropy S |
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) Non-Flammable |
| R-phrases | R8, R22, R50/53 |
| S-phrases | (S2), S60, S61 |
| NFPA 704 |
0
2
0
OX
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| Related compounds | |
| Other anions | Potassium manganite Potassium manganate |
| Other cations | Sodium permanganate Ammonium permanganate |
| Related compounds | Manganese heptoxide |
| (verify) (what is: /?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) |
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| Infobox references | |
Potassium permanganate is an inorganic chemical compound with the formula KMnO4. It is a salt consisting of K+ and MnO4− ions. Formerly known as permanganate of potash or Condy's crystals, it is a strong oxidizing agent. It dissolves in water to give intensely purple solutions, the evaporation of which leaves 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.
Contents |
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 a source of oxygen, like potassium nitrate or chlorate.[2] This process gives potassium manganate, which upon electrolytic oxidation in alkaline media, or by boiling the manganate solution in the presence of carbon dioxide until all the green color is discharged, gives potassium permanganate.[3]
or:
In which the potassium permanganate is separated by filtering the insoluble manganese dioxide, evaporating the solution to 1/3 and recrystallizing it.
Permanganate salts can also be generated by treating a solution of Mn2+ ions with strong oxidants such as lead dioxide (PbO2), or sodium bismuthate (NaBiO3). Tests for the presence of manganese exploit the vivid violet colour of permanganate produced by these reagents.
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.[4] In the solid (as in solution), each MnO4- centres are tetrahedral. The Mn-O distances are 1.62 Å.[5]
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.[6]
Aqueous, acidic solutions of KMnO4 are used to collect gaseous mercury in flue gas during stationary source emissions testing.[7]
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. 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:[8]
Potassium permanganate oxidizes aldehydes to carboxylic acids, such as the conversion of n-heptanal to heptanoic acid:[9]
Even an alkyl group (with a benzylic hydrogen) on an aromatic ring is oxidized, e.g. toluene to benzoic acid.[10]
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 colour 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.[11] Its sterilizing properties are another reason for inclusion of KMnO4 in a survival kit.
Purple benzene refers to solutions prepared by treating a two phase mixture of aqueous potassium permanganate and benzene with a quat salt. The quat cation forms a salt of permanganate that is soluble in benzene, giving this organic solvent a purple color. The reaction illustrate the ability of quat salts to confer lipophilicity to hydrophilic anions. The same effect can be obtained by complexing the potassium to 18-crown-6.[12]
Concentrated sulfuric acid reacts with KMnO4 to give Mn2O7, which can be explosive.[13] 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.
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.[14][15]
Potassium permanganate decomposes when exposed to light:
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 ( a lemon rubbed into the skin works especially well), with sodium thiosulfate or with a weak solution of hydrogen peroxide.
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 principal chemicals used 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.[16] It was used on props and sets in films such as "Troy", "300" and "Indiana Jones".
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,[17][18] and fungal infections of the hands or feet.[19] 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, is used additionally to treat waste water. Historically it was used to disinfect drinking water.[20][21] It currently finds application in the control of nuisance organisms such as Zebra mussels in fresh water collection and treatment systems.[22]
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.[23][24] Permanganate washes were once used to treat gonorrhea[25] and are still used to treat candidiasis.[26] It can be used to inactivate the poison strychnine.
Aside from its use in water treatment, the other major application of KMnO4 is as a reagent for the synthesis of organic compounds.[27] Significant amounts are required for the synthesis of ascorbic acid, chloramphenicol, saccharin, isonicotinic acid, and pyrazinoic acid.[2]
Ethylene absorbents extend storage time of banana even at high temperatures. This effect can be exploited by packing banana in a polyethylene with potassium permanganate as an ethylene absorbent, that doubles banana lifespan up to 3–4 weeks without the need for refrigeration.[28][29][30]
Potassium permanganate is typically included in survival kits: as a fire starter,[11] water sterilizer and for creating distress signals on snow.[31]
In 1659, Johann Rudolf 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 and then finally red. This report represents the first description of the production of potassium permanganate.[32] 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.[33]
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.
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 or other simple alcohols it will result in a violent combustion reaction.
Potassium permanganate may leave behind a brownish stain which can be removed by using sodium bisulfite or oxalic acid.
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