Potassium cyanide

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
Names
IUPAC name
Potassium cyanide
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
ECHA InfoCard 100.005.267
EC Number
  • 205-792-3
RTECS number
  • TS8750000
UNII
UN number 1680
Properties
KCN
Molar mass 65.12 g/mol
Appearance White crystalline solid
deliquescent
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
1.410
Thermochemistry
127.8 J K−1 mol−1
−131.5 kJ/mol
Hazards
Safety data sheet ICSC 0671
GHS pictograms
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 code
0
4
0
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)[1]
NIOSH (US health exposure limits):
PEL (Permissible)
 TWA 5 mg/m3[2]
REL (Recommended)
 C 5 mg/m3 (4.7 ppm) [10-minute][2]
IDLH (Immediate danger)
 25 mg/m3[2]
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).
N verify (what is YN ?)
Infobox references

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 similar to lye.[7]

Production

KCN is produced by treating hydrogen cyanide with an aqueous solution of potassium hydroxide, followed by evaporation of the solution in a vacuum:[8]

HCN + KOH → KCN + H2O

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:[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. 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. [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 the less toxic 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)).

Toxicity
Main article: Cyanide poisoning

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 LD50 is estimated at 140 mg.[13]

A number of prominent persons who were killed or died by suicide using potassium cyanide include members of the Young Bosnia and infamous personalities in Nazi Germany, such as Erwin Rommel, Hitler's longtime companion Eva Braun, Joseph Goebbels, Heinrich Himmler, and Hermann Göring. It was also used by World War II–era British agents (as purpose-made suicide pills), computer scientist Alan Turing, polymer chemist Wallace Carothers, and 19th-century German chemist Viktor Meyer,[14] and in various religious cult suicides such as by the Peoples Temple, Danish writer Gustav Wied in 1914, members of the LTTE involved in the assassination of Indian prime minister Rajiv Gandhi, Bengal's first woman martyr Pritilata Waddedar, Jason Altom, who was a promising graduate student in the lab of Nobel Prize–winning chemist EJ Corey at Harvard, and John B. Mclemore, an Alabamian man whose life and 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.[15]

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

References
  1. "Cyanides (as CN)". Immediately Dangerous to Life and Health Concentrations (IDLH). National Institute for Occupational Safety and Health (NIOSH).
  2. NIOSH Pocket Guide to Chemical Hazards. "#0522". National Institute for Occupational Safety and Health (NIOSH).
  3. "POTASSIUM CYANIDE | CAMEO Chemicals | NOAA".
  4. 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". 8 July 2006.
  6. Online Mendelian Inheritance in Man (OMIM): 304300
  7. "The only taste: Cyanide is acrid". hindustantimes.com. Hindustan Times. 8 July 2006.
  8. Pradyot Patnaik. Handbook of Inorganic Chemicals. McGraw-Hill, 2002, ISBN 0-07-049439-8
  9. Von Wagner, Rudolf (1897). Manual of chemical technology. New York: D. Appleton & Co. p. 474 & 477.
  10. Crystallography Open Database, Structure of KCN
  11. 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 (Submitted manuscript). 47 (17): 11082–11092. doi:10.1103/PhysRevB.47.11082..
  12. J. Towler, MD. "The Silver Sunbeam (Facsimile 1864 edition, 1969)" pg 119
  13. John Harris Trestrail III. Criminal Poisoning - Investigational Guide for Law Enforcement, Toxicologists, Forensic Scientists, and Attorneys (2nd edition). p. 119
  14. "Top 10 Scientists who Committed Suicide". 7 October 2007.
  15. "War criminal 'took cyanide' in Hague court". BBC News. 1 December 2017. Retrieved 1 December 2017.
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