Chemistry:Potassium cyanide

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Potassium cyanide
IUPAC name
Potassium cyanide
3D model (JSmol)
EC Number
  • 205-792-3
RTECS number
  • TS8750000
UN number 1680
Molar mass 65.12 g/mol
Appearance White crystalline solid
Odor faint, 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 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/100mL
Solubility in hydroxylamine 41 g/100 mL
Acidity (pKa) 11.0
−37.0·10−6 cm3/mol
127.8 J K−1 mol−1
−131.5 kJ/mol
Safety data sheet ICSC 0671
GHS pictograms GHS05: CorrosiveGHS06: ToxicGHS08: Health hazardGHS09: Environmental hazard
GHS Signal word Danger
H290, H300, H310, H330, H370, H372, H410
P260, P264, P273, P280, P284, P301+310
NFPA 704 (fire diamond)
Flammability code 0: Will not burn. E.g. waterHealth code 4: Very short exposure could cause death or major residual injury. E.g. VX gasReactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no codeNFPA 704 four-colored diamond
Flash point Non-flammable
Lethal dose or concentration (LD, LC):
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]
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).
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Infobox references

Potassium cyanide is a compound with the formula KCN. This colorless crystalline salt, similar in appearance to sugar, 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. The moist solid emits small amounts of hydrogen cyanide due to hydrolysis, which smells like bitter almonds.[5] Not everyone, however, can smell this; the ability to do so is a genetic trait.[6]

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


KCN is produced by treating hydrogen cyanide with an aqueous solution of potassium hydroxide, followed by evaporation of the solution in a vacuum:[9]


About 50,000 tons of potassium cyanide are produced yearly.[4]

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:[10]

K4[Fe(CN)6] → 4 KCN + FeC2 + N2


In aqueous solution, KCN is dissociated into hydrated potassium (K+) ions and cyanide (CN) ions. The common form of solid KCN, stable at ambient pressure and temperature, has the same cubic crystal structure as sodium chloride, with each potassium ion surrounded by six cyanide ions, and vice versa. Despite the cyanide ions being diatomic, and thus less symmetric than chloride, they rotate so rapidly, 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 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.[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 for sodium thiosulfate, but KCN is still used. It was extensively used by high ranking Nazi officials to commit suicide in the last days of World War II, such as Hermann Göring, who took a capsule the night before his execution.

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)).


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 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.[14]

A number of prominent persons who were killed or died by suicide using potassium cyanide include:

  • Viktor Meyer, 19th-century German chemist, died by suicide in 1897 after taking cyanide[15]
  • Gustav Wied, Danish novelist, poet, and playwright, in 1914
  • Pritilata Waddedar, Bengal's first female martyr, took cyanide in 1932 to avoid capture by British police
  • Badal Gupta, a revolutionary from Bengal, who is noted for launching an attack on the Writers' Building in Kolkata, consumed cyanide in 1930 immediately after the attack. He was martyred at the age of 18.
  • Wallace Carothers, polymer chemist who died by suicide in 1937 after battling depression for years
  • Infamous personalities in Nazi Germany, such as Erwin Rommel, Hitler's longtime companion Eva Braun, Joseph Goebbels, Heinrich Himmler, and Hermann Göring
  • World War II–era British agents (as purpose-made suicide pills)
  • 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
  • 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
  • John B. Mclemore, an Alabamian man whose life and 2015 suicide were the subject of the popular podcast "S-Town"
  • Slobodan Praljak, a wartime general in Republic of Croatia and convicted war criminal, died by suicide by drinking from a vial containing potassium cyanide during the reading of his sentence in The Hague on International Criminal Tribunal for the former Yugoslavia (ICTY) on 29 November 2017.[16]

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. Such solutions should be kept alkaline whenever possible so as to eliminate the possibility of generation of hydrogen cyanide:[4]

KCN + H2O2 → KOCN + H2O


  1. 1.0 1.1 1.2 NIOSH Pocket Guide to Chemical Hazards. "#0522". National Institute for Occupational Safety and Health (NIOSH). 
  2. "Cyanides (as CN)". Immediately Dangerous to Life and Health Concentrations (IDLH). National Institute for Occupational Safety and Health (NIOSH). 
  4. 4.0 4.1 4.2 4.3 Andreas Rubo, Raf Kellens, Jay Reddy, Joshua Wooten, Wolfgang Hasenpusch "Alkali Metal Cyanides" in Ullmann's Encyclopedia of Industrial Chemistry 2006 Wiley-VCH, Weinheim, Germany. doi:10.1002/14356007.i01_i01
  5. "Suicide note reveals taste of cyanide". 2006-07-08. 
  6. Online Mendelian Inheritance in Man (OMIM) 304300
  7. ലേഖകൻ, മാധ്യമം (2021-12-19). "'സയനൈഡ് ചവർപ്പാണ്... പുകച്ചിലാണ്...'; ആ 'രുചി രഹസ്യം' പുറത്തുവിട്ട മലയാളി നടന്ന വഴിയിലൂടെ | Madhyamam" (in ml). 
  8. "The only taste: Cyanide is acrid". Hindustan Times. 2006-07-08. 
  9. Pradyot Patnaik. Handbook of Inorganic Chemicals. McGraw-Hill, 2002, ISBN:0-07-049439-8
  10. Von Wagner, Rudolf (1897). Manual of chemical technology. New York: D. Appleton & Co.. p. 474 & 477.;view=1up;seq=502. 
  11. Crystallography Open Database, Structure of KCN
  12. H. T. Stokes; D. L. Decker; H. M. Nelson; J. D. Jorgensen (1993). "Structure of potassium cyanide at low temperature and high pressure determined by neutron diffraction". Physical Review B 47 (17): 11082–11092. doi:10.1103/PhysRevB.47.11082. PMID 10005242. Bibcode1993PhRvB..4711082S. .
  13. J. Towler, MD. "The Silver Sunbeam (Facsimile 1864 edition, 1969)" pg 119
  14. John Harris Trestrail III. Criminal Poisoning - Investigational Guide for Law Enforcement, Toxicologists, Forensic Scientists, and Attorneys (2nd edition). p. 119
  15. "Top 10 Scientists who Committed Suicide". 2007-10-07. 
  16. "War criminal 'took cyanide' in Hague court" (in en-GB). BBC News. 2017-12-01. 

External links