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Potassium cyanide
Chemical compound

Potassium cyanide (KCN) is a colorless, sugar-like salt highly soluble in water and primarily used in gold mining, organic synthesis, and electroplating. It also serves smaller roles in jewellery for gilding and buffing. Highly toxic, a dose of 200–300 mg can be fatal to humans. When moist, it emits hydrogen cyanide through hydrolysis, which smells like bitter almonds. Its taste is described as acrid and bitter, with a burning sensation similar to lye.

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Production

KCN is produced by treating hydrogen cyanide with an aqueous solution of potassium hydroxide, followed by evaporation of the solution in a vacuum:6

HCN + KOH → KCN + H2O

About 50,000 tons of potassium cyanide are produced yearly.7 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.8 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. 1011

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)).

Analytical chemistry

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.16

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 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.17

Disposal

Due to toxicity considerations, the disposal of cyanide is subject to stringent regulations. Industrial cyanide effluent is typically destroyed by oxidation using peroxysulfuric acid, hydrogen peroxide, sulfur dioxide/copper salts ("Inco process") or all three ("Combiox Process"). Use of sodium 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 of hydrogen cyanide.18

References

  1. Gail, Ernst; Gos, Stephen; Kulzer, Rupprecht; Lorösch, Jürgen; Rubo, Andreas; Sauer, Manfred; Kellens, Raf; Reddy, Jay; Steier, Norbert; Hasenpusch, Wolfgang (2011). "Cyano Compounds, Inorganic". Ullmann's Encyclopedia of Industrial Chemistry. doi:10.1002/14356007.a08_159.pub3. ISBN 978-3-527-30385-4. 978-3-527-30385-4 ↩

  2. "Suicide note reveals taste of cyanide". The Sydney Morning Herald. 8 July 2006. http://www.smh.com.au/news/world/suicide-note-reveals-taste-of-cyanide/2006/07/08/1152240534587.html ↩

  3. Not everyone, however, can smell cyanide; the ability to do so is a genetic trait.Online Mendelian Inheritance in Man (OMIM): 304300 /wiki/Genetic_trait ↩

  4. ലേഖകൻ, മാധ്യമം (19 December 2021). "'സയനൈഡ് ചവർപ്പാണ്... പുകച്ചിലാണ്...'; ആ 'രുചി രഹസ്യം' പുറത്തുവിട്ട മലയാളി നടന്ന വഴിയിലൂടെ | Madhyamam". www.madhyamam.com (in Malayalam). Retrieved 21 December 2021. https://www.madhyamam.com/lifestyle/men/about-the-experience-of-malayalee-mp-prasad-who-drank-cyanide-895697 ↩

  5. "The only taste: Cyanide is acrid". hindustantimes.com. Hindustan Times. 8 July 2006. http://www.hindustantimes.com/india/the-only-taste-cyanide-is-acrid/story-vhsbYsiNyWzIfakN4HBK0H.html ↩

  6. Gail, Ernst; Gos, Stephen; Kulzer, Rupprecht; Lorösch, Jürgen; Rubo, Andreas; Sauer, Manfred; Kellens, Raf; Reddy, Jay; Steier, Norbert; Hasenpusch, Wolfgang (2011). "Cyano Compounds, Inorganic". Ullmann's Encyclopedia of Industrial Chemistry. doi:10.1002/14356007.a08_159.pub3. ISBN 978-3-527-30385-4. 978-3-527-30385-4 ↩

  7. Gail, Ernst; Gos, Stephen; Kulzer, Rupprecht; Lorösch, Jürgen; Rubo, Andreas; Sauer, Manfred; Kellens, Raf; Reddy, Jay; Steier, Norbert; Hasenpusch, Wolfgang (2011). "Cyano Compounds, Inorganic". Ullmann's Encyclopedia of Industrial Chemistry. doi:10.1002/14356007.a08_159.pub3. ISBN 978-3-527-30385-4. 978-3-527-30385-4 ↩

  8. Gail, Ernst; Gos, Stephen; Kulzer, Rupprecht; Lorösch, Jürgen; Rubo, Andreas; Sauer, Manfred; Kellens, Raf; Reddy, Jay; Steier, Norbert; Hasenpusch, Wolfgang (2011). "Cyano Compounds, Inorganic". Ullmann's Encyclopedia of Industrial Chemistry. doi:10.1002/14356007.a08_159.pub3. ISBN 978-3-527-30385-4. 978-3-527-30385-4 ↩

  9. Von Wagner, Rudolf (1897). Manual of chemical technology. New York: D. Appleton & Co. pp. 474 & 477. http://babel.hathitrust.org/cgi/pt?id=uc2.ark:/13960/t3tt4gz1p;view=1up;seq=502 ↩

  10. Crystallography Open Database, Structure of KCN http://www.crystallography.net/5910129.html ↩

  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. Bibcode:1993PhRvB..4711082S. doi:10.1103/PhysRevB.47.11082. PMID 10005242.. https://zenodo.org/record/1233725 ↩

  12. J. Towler, MD. "The Silver Sunbeam (Facsimile 1864 edition, 1969)" p. 119 ↩

  13. Crookes, William, ed. (10 September 1858). "Photographic Notes and Queries". The Photographic News: A Weekly Record of the Progress of Photography. 1 (1). London: Cassell, Petter, and Galpin: 10–12. https://books.google.com/books?id=pvU_AAAAYAAJ&q=%22cyanogen+soap%22&pg=PA11 ↩

  14. Reports of Trials for Murder by Poisoning; by Prussic Acid, Strychnia, Antimony, Arsenic, and Aconita. Including the trials of Tawell, W. Palmer, Dove, Madeline Smith, Dr. Pritchard, Smethurst, and Dr. Lamson, with chemical introduction and notes on the poisons used, G. Lathom Browne and C. G. Stewart, London: Stevens and Sons, 1883; redistributed by Project Gutenberg. https://www.gutenberg.org/ebooks/50636 ↩

  15. "Cyanuret of potassium", Paper, Shadows and Light, Robert Douglas. Accessed 2024-01-20. https://www.papershadowsandlight.com/cyanuretofpotassium ↩

  16. Pawlowski, Lucjan (March 1994). "Standard methods for the examination of water and wastewater, 18th edition". Science of the Total Environment. 142 (3): 227–228. Bibcode:1994ScTEn.142..227P. doi:10.1016/0048-9697(94)90332-8. ISSN 0048-9697. https://doi.org/10.1016/0048-9697(94)90332-8 ↩

  17. John Harris Trestrail III. Criminal Poisoning –Investigational Guide for Law Enforcement, Toxicologists, Forensic Scientists, and Attorneys (2nd ed.) p. 119 ↩

  18. Gail, Ernst; Gos, Stephen; Kulzer, Rupprecht; Lorösch, Jürgen; Rubo, Andreas; Sauer, Manfred; Kellens, Raf; Reddy, Jay; Steier, Norbert; Hasenpusch, Wolfgang (2011). "Cyano Compounds, Inorganic". Ullmann's Encyclopedia of Industrial Chemistry. doi:10.1002/14356007.a08_159.pub3. ISBN 978-3-527-30385-4. 978-3-527-30385-4 ↩