Potassium permanganate
 Different sizes of potassium permanganate crystals.
| |
| Names | |
|---|---|
| IUPAC name
Potassium manganate(VII)
| |
| Other names
Chameleon mineral
Condy's crystals Hypermangan Permanganate of potash | |
| Properties | |
| KMnO4 | |
| Molar mass | 158.034 g/mol |
| Appearance | Dark purple crystals |
| Odor | Odorless |
| Density | 2.703 g/cm3 (25 °C) |
| Melting point | 240 °C (464 °F; 513 K) (decomposes) |
| Boiling point | Decomposes |
| 6.4 g/100 ml (20 °C) 25 g/100 ml (65 °C) | |
| Solubility | Reacts with acids, alcohols, glycerol, halogens Soluble in glacial acetic acid, anhydrous acetonitrile, benzonitrile, pyridine, sulfolane, trifluoroacetic acid Insoluble in hydrocarbons, halocarbons |
| Solubility in bromoacetic acid | 1.02 g/100 ml |
| Vapor pressure | ~ 0 mmHg |
| Thermochemistry | |
| Std molar
entropy (S |
171.7 J·mol-1·K-1 |
| Std enthalpy of
formation (ΔfH |
−813.4 kJ/mol |
| Hazards | |
| Safety data sheet | Sigma-Aldrich |
| Flash point | Non-flammable |
| Lethal dose or concentration (LD, LC): | |
| LD50 (Median dose)
|
1,090 mg/kg (oral, rat) |
| Related compounds | |
| Related compounds
|
Potassium manganate |
| Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). | |
| Infobox references | |
Potassium permanganate is an ionic compound with the formula KMnO4. At room temperature and pressure it is a deep violet crystalline salt that dissolves to form an intense violet or magenta solution, often tending to black in high concentrations. Potassium permanganate contains manganese in the +7 oxidation state, and is a strong oxidizer commonly encountered in a lab setting. It participates in a wide range of reactions, from simple colour change demonstrations to complex organic syntheses.
Contents
Properties
Chemical
Potassium permanganate in the presence of strong acids will oxidize primary alcohols to carboxylic acids and secondary alcohols to ketones.
When exposed to strong light, potassium permanganate will break down to form potassium manganate and manganese dioxide.
- 2 KMnO4 → K2MnO4 + MnO2 + O2
Concentrated sulfuric acid dehydrates potassium permanganate to the incredibly unstable manganese heptoxide.
- 2 KMnO4 + 2 H2SO4 → Mn2O7 + KHSO4 + H2O
Potassium permanganate reacts with concentrated hydrochloric acid to give chlorine and manganese(II) chloride:
- 2 KMnO4 + 16 HCl2 → 2 KCl + 2 MnCl2 + 5 Cl2 + 8 H2O
Adding a few drops of glycerol (glycerin) to potassium permanganate creates an intense flame hot enough to light thermite and pyrotechnics. Addition of a dilute solution of sodium hydroxide and sucrose to potassium manganate solution and hydrous manganese dioxide creates a "chemical chameleon" reaction, in which the permanganate ion is slowly reduced several times, producing several vivid color changes.
In acidic solutions, permanganates reduce to manganese(II) ions. In neutral and mildly basic solutions, it reduces to manganese dioxide. In extremely basic solutions, the bluish-green manganate ion would form, but it would exist only transiently as it would be reduced to manganese dioxide.
Physical
Potassium permanganate is a dark purple, almost black, crystalline solid that is soluble in water to form an intense magenta solution.
Availability
Like many chemicals, potassium permanganate was easier to find a few years ago, moderately difficult to find today and most likely getting harder and harder to locate in the next few years. It is sometimes sold as a camping chemical because it can light hot fires (when combined with glycerin), decontaminate water and disinfect wounds. Today it is easiest to find potassium permanganate at pharmacies/chemists or in aquarium supply stores as a type of water treatment.
Legality
The sale of potassium permanganate is regulated in several countries, due to its use in cocaine purification and methcathinone production. Check the local laws before using this substance.
Preparation
Making permanganate is a difficult and messy exercise, and although extensive discussion on this topic has been undertaken on Sciencemadness, a method that is accessible to the amateur but also produces worthwhile yields is still yet to be found.
The most popular method involves fusing a mixture of potassium nitrate, potassium hydroxide and manganese dioxide to form crude potassium manganate, then disproportionating it (part of it oxidizes the other part, and is reduced as a result) to permanganate and manganese dioxide, with the addition of a dilute solution of sodium bicarbonate (baking soda). The yield of this route is poor, but the reagents are cheap.
A simple method involves fusing managnese dioxide, potassium chlorate and potassium hydroxide at around 400 °C for a while. Elemental chlorine is used to turn the manganate into permanganate. The yield of this route isn't great, but it's somewhat better than the previous ones mentioned.[1]
- 3 MnO2 + 6 KOH + KClO3 → 3 K2MnO4 + KCl + 3 H2O
- K2MnO4 + ½ Cl2 → KMnO4 + KCl
Projects
- Oxidize alcohols to carboxylic acids and ketones
- Make benzoic acid from toluene
- Convert alkenes into diols
- Ignition of glycerol demonstration
- Disinfectant
- Age paper and wood
Handling
Safety
While not overly toxic, potassium permanganate may strongly stain skin or other materials due to the formation of manganese dioxide when reacted with certain organics.
Contact with concentrated sulfuric acid will create a very powerful, unpredictable and unstable oxidizer manganese heptoxide. Accidental creation of this must be avoided, and has been known to create disaster for the backyard chemist.
Potassium permanganate vigorously reacts with many organic compounds, such as glycerol and ethylene glycol, causing some to spontaneously combust. Appropriate care must be taken when combining the two in any setting.
Storage
Potassium permanganate should be stored in closed bottles, away from any acidic vapors or volatile organic compounds. KMnO4 solutions should be covered as they will slowly break down in open air, due to dust or chemical vapors, generally organic.
Disposal
Potassium permanganate can be neutralized with hydrogen peroxide, resulting potassium hydroxide and manganese dioxide. The latter can be recycled through sedimentation or filtration.
A mild reducing agent can also be used.
Gallery
Crystals of potassium permanganate
