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| Names | |
|---|---|
| IUPAC name Potassium cyanide | |
| Identifiers | |
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3D model (JSmol) | |
| ChEBI | |
| ChemSpider |
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| ECHA InfoCard | 100.005.267 |
| EC Number |
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| RTECS number |
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| UNII | |
| UN number | 1680 |
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| Properties | |
| KCN | |
| Molar mass | 65.12 g/mol |
| Appearance | White crystalline solid deliquescent |
| Odor | faint,bitter almond-like |
| Density | 1.52 g/cm3 |
| Melting point | 634.5 °C (1,174.1 °F; 907.6 K) |
| Boiling point | 1,625 °C (2,957 °F; 1,898 K) |
| 71.6 g/100 ml (25 °C) 100 g/100 ml (100 °C) | |
| Solubility inmethanol | 4.91 g/100 ml (20 °C) |
| Solubility inglycerol | soluble |
| Solubility informamide | 14.6 g/100 ml |
| Solubility inethanol | 0.57 g/100 ml |
| Solubility inhydroxylamine | 41 g/100 ml |
| Acidity (pKa) | 11.0 |
| −37.0·10−6 cm3/mol | |
Refractive index (nD) | 1.410 |
| Thermochemistry | |
Std molar entropy(S⦵298) | 127.8 J K−1 mol−1 |
Std enthalpy of formation(ΔfH⦵298) | −131.5 kJ/mol |
| Hazards | |
| GHS labelling: | |
    | |
| Danger | |
| H290,H300,H310,H330,H370,H372,H410 | |
| P260,P264,P273,P280,P284,P301+P310 | |
| NFPA 704 (fire diamond) | |
| Flash point | Non-flammable |
| Lethal dose or concentration (LD, LC): | |
LD50 (median dose) | 5 mg/kg (oral, rabbit) 10 mg/kg (oral, rat) 5 mg/kg (oral, rat) 8.5 mg/kg (oral, mouse)[2] |
| NIOSH (US health exposure limits): | |
PEL (Permissible) | TWA 5 mg/m3[1] |
REL (Recommended) | C 5 mg/m3 (4.7 ppm) [10-minute][1] |
IDLH (Immediate danger) | 25 mg/m3[1] |
| Safety data sheet (SDS) | ICSC 0671 |
| Related compounds | |
Otheranions | Potassium cyanate Potassium thiocyanate |
Othercations | Sodium cyanide Rubidium cyanide lithium cyanide caesium cyanide |
Related compounds | Hydrogen cyanide |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Potassium cyanide is a compound with theformulaKCN. It is a colorless salt, similar in appearance tosugar, that is highlysoluble in water. Most KCN is used ingold mining,organic synthesis, andelectroplating. Smaller applications includejewellery for chemicalgilding andbuffing.[4] Potassium cyanide is highly toxic, and a dose of 200 to 300 milligrams will kill nearly any human.
The moist solid emits small amounts ofhydrogen cyanide due tohydrolysis (reaction with water). Hydrogen cyanide is often described as having an odor resembling that ofbitter almonds.[5][6]
The taste of potassium cyanide has been described as acrid and bitter, with a burning sensation similar to lye.[7] Potassium cyanide kills so rapidly its taste hasn't been reliably documented, in 2006 anIndian man named MP Prasad committed suicide using potassium cyanide. He was agoldsmith and was aware of the mystery behind its taste. In the suicide note Prasad left, the final words written were that potassium cyanide "burns the tongue and tastes acrid", for obvious reasons this description hasn't been independently confirmed.[8][9]
KCN is produced by treatinghydrogen cyanide with an aqueous solution ofpotassium hydroxide, followed by evaporation of the solution in a vacuum:[4]
About 50,000 tons of potassium cyanide are produced yearly.[4] For laboratory purpose it is easier to pass hydrogen cyanide through an alcoholic solution of potassium base because the crystals of potassium cyanide are not soluble in alcohol .
Before 1900 and the invention of theCastner process, potassium cyanide was the most important source ofalkali metalcyanides.[4] In this historical process, potassium cyanide was produced by decomposingpotassium ferrocyanide:[10]
In aqueous solution, KCN is dissociated into hydrated potassium (K+) ions andcyanide (CN−) ions. As a solid, KCN has structure resemblingsodium chloride: with each potassium ion surrounded by six cyanide ions, and vice versa. Despite being diatomic, and thus less symmetric than chloride, the cyanide ions rotate so rapidly that their time-averaged shape is spherical. At low temperature and high pressure, this free rotation is hindered, resulting in a less symmetric crystal structure with the cyanide ions arranged in sheets.[11][12]
KCN andsodium cyanide (NaCN) are widely used inorganic synthesis for the preparation ofnitriles andcarboxylic acids, particularly in thevon Richter reaction. It also finds use for the synthesis ofhydantoins, which can be useful synthetic intermediates, when reacted with a carbonyl compound such as analdehyde orketone in the presence ofammonium carbonate.
KCN is used as aphotographic fixer in the wet platecollodion process.[13] The KCN dissolves silver where it has not been made insoluble by the developer. This reveals and stabilizes the image, making it no longer sensitive to light. Modern wet plate photographers may prefer less toxic fixers, often opting forsodium thiosulfate, but KCN is still used.
In the 19th century, cyanogensoap, a preparation containing potassium cyanide, was used byphotographers to removesilver stains from their hands.[14]: 11 [15]: 73 [16]
Ingold mining, KCN forms the water-soluble saltpotassium gold cyanide (or gold potassium cyanide) and potassium hydroxide from gold metal in the presence of oxygen (usually from the surrounding air) and water:
A similar process uses NaCN to producesodium gold cyanide (NaAu(CN2)).
In analytical chemistry, potassium cyanide is used as complexing agent for chemical analysis of zinc in water and wastewater. The cyanide group complexes zinc and other heavy metals, which is later extracted and analyzed in a spectro-photometer.[17]
Potassium cyanide is a potent inhibitor ofcellular respiration, acting on mitochondrialcytochrome c oxidase, hence blockingoxidative phosphorylation.Lactic acidosis then occurs as a consequence ofanaerobic metabolism. Initially, acute cyanide poisoning causes a red or ruddy complexion in the victim because the tissues are not able to use theoxygen in the blood. The effects of potassium cyanide and sodium cyanide are identical, and symptoms of poisoning typically occur within a few minutes of ingesting the substance: the person loses consciousness, andbrain death eventually follows. During this period the victim may sufferconvulsions. Death is caused byhistotoxic hypoxia/cerebral hypoxia. The expected LD100 dose (human) for potassium cyanide is 200–300 mg while the median lethal dose LD50 is estimated at 140 mg.[18]
Due to toxicity considerations, the disposal of cyanide is subject to stringent regulations. Industrial cyanide effluent is typically destroyed by oxidation usingperoxysulfuric acid,hydrogen peroxide,sulfur dioxide/copper salts ("Inco process") or all three ("Combiox Process"). Use ofsodium hypochlorite, traditional for laboratory-scale wastes, is impractical on a commercial scale. Hydrolysis at higher temperatures is highly effective, but requires specialized equipment. Lastly, cyanide wastes can be acidified for recovery ofhydrogen cyanide.[4]
