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Potassium cyanide
Names
	IUPAC name Potassium cyanide
Identifiers
CAS Number	151-50-8 ;
3D model (JSmol)	Interactive image;
ChEBI	CHEBI:33191 ;
ChemSpider	8681 ;
ECHA InfoCard	100.005.267
EC Number	205-792-3;
PubChem CID	9032;
RTECS number	TS8750000;
UNII	MQD255M2ZO ;
UN number	1680
CompTox Dashboard (EPA)	DTXSID0024268 ;
	InChI InChI=1S/CN.K/c1-2;/q-1;+1 Key: NNFCIKHAZHQZJG-UHFFFAOYSA-N ; InChI=1/CN.K/c1-2;/q-1;+1 Key: NNFCIKHAZHQZJG-UHFFFAOYAH;
	SMILES [K+].[C-]#N;
Properties
Chemical formula	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)
Solubility in water	71.6 g/100 ml (25 °C) ; 100 g/100 ml (100 °C)
Solubility in methanol	4.91 g/100 ml (20 °C)
Solubility in glycerol	soluble
Solubility in formamide	14.6 g/100 ml
Solubility in ethanol	0.57 g/100 ml
Solubility in hydroxylamine	41 g/100 ml
Acidity (pKa)	11.0
Magnetic susceptibility (χ)	−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:
Pictograms
Signal word	Danger
Hazard statements	H290, H300, H310, H330, H370, H372, H410
Precautionary statements	P260, P264, P273, P280, P284, P301+P310
NFPA 704 (fire diamond)	4 0 0
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)
	NIOSH (US health exposure limits):
PEL (Permissible)	TWA 5 mg/m3
REL (Recommended)	C 5 mg/m3 (4.7 ppm) [10-minute]
IDLH (Immediate danger)	25 mg/m3
Safety data sheet (SDS)	ICSC 0671
Related compounds
Other anions	Potassium cyanate; Potassium thiocyanate
Other cations	Sodium cyanide; Rubidium cyanide; lithium cyanide; caesium cyanide
Related compounds	Hydrogen cyanide
	Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). verify (what is ?) Infobox references

Potassium cyanide is a compound with the formula KCN. It is a colorless salt, similar in appearance to sugar, that is highly soluble in water. Most KCN is used in gold mining, organic synthesis, and electroplating. Smaller applications include jewellery for chemical gilding and buffing.^[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 of hydrogen cyanide due to hydrolysis (reaction with water). Hydrogen cyanide is often described as having an odor resembling that of bitter almonds.^[5]^[6]

The taste of potassium cyanide has been described as acrid and bitter, with a burning sensation^[7]^{[unreliable source?]} similar to lye.^[8]

Production

KCN is produced by treating hydrogen cyanide with an aqueous solution of potassium hydroxide, followed by evaporation of the solution in a vacuum:^[4]

HCN + KOH → KCN + H₂O

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 .

Historical production

Before 1900 and the invention of the Castner process, potassium cyanide was the most important source of alkali metal cyanides.^[4] In this historical process, potassium cyanide was produced by decomposing potassium ferrocyanide:^[9]

K₄[Fe(CN)₆] → 4 KCN + FeC₂ + N₂

Structure

In aqueous solution, KCN is dissociated into hydrated potassium (K⁺) ions and cyanide (CN⁻) ions. As a solid, KCN has structure resembling sodium 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. ^[10]^[11]

Applications

KCN and sodium cyanide (NaCN) are widely used in organic synthesis for the preparation of nitriles and carboxylic acids, particularly in the von Richter reaction. It also finds use for the synthesis of hydantoins, which can be useful synthetic intermediates, when reacted with a carbonyl compound such as an aldehyde or ketone in the presence of ammonium carbonate.

KCN is used as a photographic fixer in the wet plate collodion process.^[12] 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 for sodium thiosulfate, but KCN is still used.

In the 19th century, cyanogen soap, a preparation containing potassium cyanide, was used by photographers to remove silver stains from their hands.^[13]^: 11^[14]^: 73^[15]

Potassium gold cyanide

In gold mining, KCN forms the water-soluble salt potassium gold cyanide (or gold potassium cyanide) and potassium hydroxide from gold metal in the presence of oxygen (usually from the surrounding air) and water:

4 Au + 8 KCN + O₂ + 2 H₂O → 4 K[Au(CN)₂] + 4 KOH

A similar process uses NaCN to produce sodium gold cyanide (NaAu(CN₂)).

Toxicity

Potassium cyanide is a potent inhibitor of cellular respiration, acting on mitochondrial cytochrome c oxidase, hence blocking oxidative phosphorylation. Lactic acidosis then occurs as a consequence of anaerobic metabolism. Initially, acute cyanide poisoning causes a red or ruddy complexion in the victim because the tissues are not able to use the oxygen 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, and brain death eventually follows. During this period the victim may suffer convulsions. Death is caused by histotoxic 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.^[16]

People who killed themselves, were killed, or killed someone else using potassium cyanide include:

Viktor Meyer, 19th-century German chemist, died by suicide in 1897 after taking cyanide^[17]
Gustav Wied, Danish novelist, poet, and playwright, in 1914
Pritilata Waddedar, an Indian revolutionary nationalist, took cyanide in 1932 to avoid capture by Indian Imperial Police, British India
Badal Gupta, a revolutionary from Bengal, who launched an attack on the Writers' Building in Kolkata, consumed cyanide in 1930 immediately after the attack.
Wallace Carothers, polymer chemist who died by suicide in 1937 after battling depression for years
Senior figures in Nazi Germany, such as Erwin Rommel, Hitler's longtime companion Eva Braun, Joseph Goebbels, Heinrich Himmler, and Hermann Göring
Alan Turing, a computer scientist who died of cyanide poisoning in 1954
Ronald Clark O'Bryan, a Texas optician who killed his son by lacing a pixy stick with potassium cyanide in 1974
Peoples Temple, the 1978 cult suicide in (Jonestown), Guyana
Members of the LTTE involved in the assassination of Indian prime minister Rajiv Gandhi in 1991
Ramon Sampedro, Spanish tetraplegic and activist whose assisted suicide in 1998 provoked a national debate about euthanasia, and was the subject of the Oscar-winning film "The Sea Inside"
Jason Altom, a promising graduate student in the lab of Nobel Prize–winning chemist EJ Corey at Harvard, died after drinking potassium cyanide in 1998
Slobodan Praljak, a wartime general in Republic of Croatia, died by suicide by drinking from a vial containing potassium cyanide during the reading of his appeal judgment in The Hague on International Criminal Tribunal for the former Yugoslavia (ICTY) on 29 November 2017.^[18]

It is used by professional entomologists as a killing agent in collecting jars, as insects succumb within seconds to the HCN fumes it emits, thereby minimizing damage to even highly fragile specimens.

KCN can be detoxified most efficiently with hydrogen peroxide or with a solution of sodium hypochlorite (NaOCl). Such solutions should be kept alkaline whenever possible so as to eliminate the possibility of generation of hydrogen cyanide:^[4]

KCN + H₂O₂ → KOCN + H₂O

KCN + NaOCl → KOCN + NaCl