View source for Potassium cyanide

{{Use dmy dates|date=June 2018}}
{{chembox
|Verifiedfields = changed
|Watchedfields = changed
|verifiedrevid = 464211374
|ImageFile1 = Potassium-cyanide-phase-I-unit-cell-3D-SF.png
|ImageFile2 = Kaliumcyanid.jpg
|IUPACName = Potassium cyanide
|Section1={{Chembox Identifiers
|ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
|ChemSpiderID = 8681
|UNII_Ref = {{fdacite|correct|FDA}}
|UNII = MQD255M2ZO
|InChI = 1/CN.K/c1-2;/q-1;+1
|InChIKey = NNFCIKHAZHQZJG-UHFFFAOYAH
|SMILES = [K+].[C-]#N
|StdInChI_Ref = {{stdinchicite|correct|chemspider}}
|StdInChI = 1S/CN.K/c1-2;/q-1;+1
|StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
|StdInChIKey = NNFCIKHAZHQZJG-UHFFFAOYSA-N
|CASNo = 151-50-8
|CASNo_Ref = {{cascite|correct|CAS}}
|EINECS = 205-792-3
|RTECS = TS8750000
|UNNumber = 1680
|ChEBI_Ref = {{ebicite|changed|EBI}}
|ChEBI = 33191
|PubChem = 9032
}}
|Section2={{Chembox Properties
|Formula = KCN
|C=1 |K=1 |N=1
|Appearance = White crystalline solid <br> [[deliquescent]]
|Odor = faint, [[Almond#Sweet and bitter almonds|bitter almond]]-like
|Density = {{val|1.52|u=g/cm3}}
|Solubility = {{ubl
 |{{val|71.6|u=g/100 ml}} ({{convert|25|C|F K}}) 
 |{{val|100|u=g/100 ml}} ({{convert|100|C|F K}})
 }}
|Solvent1 = methanol
|Solubility1 = {{val|4.91|u=g/100 ml}} ({{convert|20|C|F K}})
|Solvent2 = glycerol
|Solubility2 = soluble{{quantify|date=November 2025}}
|Solvent3 = formamide
|Solubility3 = {{val|14.6|u=g/100 ml}}
|Solvent4 = ethanol
|Solubility4 = {{val|0.57|u=g/100 ml}}
|MeltingPtC = 634.5
|BoilingPtC = 1625
|BoilingPt_notes = at {{convert|1013|hPa|kPa|disp=out}}
|BoilingPt_ref = <ref name=Sigma/>
|pKa = 11.0
|RefractIndex = 1.410
|MagSus = {{val|37.0|e=−6|u=cm3/mol}}
}}
|Section3={{Chembox Thermochemistry
|DeltaHf = {{val|-131.5|u=kJ.mol-1}}
|Entropy = {{val|127.8|u=J.K-1|s=·mol{{sup|-1}}}}
}}
|Section4={{Chembox Hazards
|ExternalSDS = [http://www.inchem.org/documents/icsc/icsc/eics0671.htm ICSC 0671]
|GHS_ref=<ref name=Sigma>{{Sigma-Aldrich |id=207810 |name=Potassium cyanide}}</ref>
|GHSPictograms = {{GHS05}}{{GHS06}}{{GHS08}}{{GHS09}}
|GHSSignalWord = Danger
|HPhrases = {{H-phrases|290|300+310+330|372|410}}
|PPhrases = {{P-phrases|234|260|262|264|270|271|273|280|284|301+310+330|302+350|304+340+310|307+311|314|362|390|391|403+233|405|406|501}}
|NFPA-H = 4
|NFPA-F = 0
|NFPA-R = 1
|NFPA-S=
|NFPA_ref=
|FlashPt = Non-flammable
|LD50 = {{ubl
 |{{val|5|u=mg/kg}} (oral, rabbit)
 |{{val|10|u=mg/kg}} (oral, rat)
 |{{val|5|u=mg/kg}} (oral, rat)
 |{{val|8.5|u=mg/kg}} (oral, mouse)<ref>{{IDLH|cyanides|Cyanides (as CN)}}</ref>
 }}
|TLV-C = {{val|5|u=mg/m3}} (skin)
|TLV-TWA = {{val|5|u=mg/m3}}
|IDLH = {{val|25|u=mg/m3}}
|REL = {{val|5|u=mg/m3}} (ceiling, {{val|4.7|u=ppm}}, 10-minute)
|PEL = {{val|5|u=mg/m3}} (TWA, skin)
|NIOSH_id=0522
}}
|Section5={{Chembox Related
|OtherAnions = {{ubl
 |[[Potassium cyanate]]
 |[[Potassium thiocyanate]]
 }}
|OtherCations = {{ubl
 |[[Hydrogen cyanide]]
 |[[Lithium cyanide]]
 |[[Sodium cyanide]]
 |[[Rubidium cyanide]]
 |[[Caesium cyanide]]
 }}
|OtherCompounds=[[Acetonitrile]]
}}
}}
'''Potassium cyanide''' is a compound with the [[chemical formula|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 chemical [[gilding]] and buffing of jewelry.<ref name=Ullmann>{{cite book|doi=10.1002/14356007.a08_159.pub3 |chapter=Cyano Compounds, Inorganic |title=Ullmann's Encyclopedia of Industrial Chemistry |date=2011 |last1=Gail |first1=Ernst |last2=Gos |first2=Stephen |last3=Kulzer |first3=Rupprecht |last4=Lorösch |first4=Jürgen |last5=Rubo |first5=Andreas |last6=Sauer |first6=Manfred |last7=Kellens |first7=Raf |last8=Reddy |first8=Jay |last9=Steier |first9=Norbert |last10=Hasenpusch |first10=Wolfgang |isbn=978-3-527-30385-4 }}</ref> Potassium cyanide is highly toxic, and a dose of {{val|200|to|300|u=milligrams}} will kill nearly any human.{{cn|date=November 2025}}

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 [[Almond#Sweet and bitter almonds|bitter almonds]].<ref>{{cite web | url=http://www.smh.com.au/news/world/suicide-note-reveals-taste-of-cyanide/2006/07/08/1152240534587.html | title=Suicide note reveals taste of cyanide| date=2006-07-08|website=The Sydney Morning Herald}}</ref><ref>Not everyone, however, can smell cyanide; the ability to do so is a [[genetic trait]].{{OMIM|304300}}</ref>

The taste of potassium cyanide has been described as acrid and bitter, with a burning sensation similar to lye.<ref>{{cite web|title=The only taste: Cyanide is acrid|url=http://www.hindustantimes.com/india/the-only-taste-cyanide-is-acrid/story-vhsbYsiNyWzIfakN4HBK0H.html|website=hindustantimes.com|publisher=Hindustan Times|date=2006-07-08}}</ref> However, potassium cyanide kills so rapidly its taste has not been reliably documented.{{cn|date=November 2025}} In 2006, an Indian man killed himself using potassium cyanide. In the suicide note he left, the final words written were that potassium cyanide "burns the tongue and tastes acrid".<ref name="Sydney Herald">{{cite news |author1=Associated Press |title=Suicide note reveals taste of cyanide |url=https://www.smh.com.au/world/suicide-note-reveals-taste-of-cyanide-20060709-gdnx7f.html |access-date=2 May 2025 |work=The Sydney Morning Herald |issue=World |publisher=Nine Entertainment Co. |date=9 July 2006}}</ref><ref name="Labatut">{{cite book |last1=Labatut |first1=Benjamín |last2=West |first2=Adrian Nathan |title=When We Cease to Understand the World |date=2020 |publisher=New York Review Books |location=New York |isbn=9781681375663 |pages=16-18 |url=https://www.abebooks.com/When-Cease-Understand-World-Labatut-Benjamin/32191017429/bd |access-date=2 May 2025 |language=English |chapter=1}}</ref>

==Production==
KCN is produced by treating [[hydrogen cyanide]] with an aqueous solution of [[potassium hydroxide]], followed by evaporation of the solution in a vacuum:<ref name=Ullmann/>
:{{chem2|HCN + KOH -> KCN + H2O}}

About {{convert|50000|ST|kg}} of potassium cyanide are produced yearly.<ref name=Ullmann/> 

===Historical production===
Before 1900 and the invention of the [[Castner process]], potassium cyanide was the most important source of [[alkali metal]] [[Sodium cyanide|cyanides]].<ref name=Ullmann /> In this historical process, potassium cyanide was produced by decomposing [[potassium ferrocyanide]]:<ref name=Wagner>{{cite book|last=Von Wagner|first=Rudolf|title=Manual of chemical technology|year=1897|publisher=D. Appleton & Co.|location=New York|pages=474 & 477|url=http://babel.hathitrust.org/cgi/pt?id=uc2.ark:/13960/t3tt4gz1p;view=1up;seq=502}}</ref>

:{{chem2|K4[Fe(CN)6] -> 4 KCN + FeC2 + N2}}

==Structure==
In aqueous solution, KCN is dissociated into hydrated potassium ({{chem2|K+}}) ions and [[cyanide]] ({{chem2|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.<ref>[http://www.crystallography.net/5910129.html Crystallography Open Database,  Structure of KCN]</ref><ref name="Stokes1993">{{cite journal | journal = Physical Review B | volume = 47 | issue = 17 | year = 1993 | pages = 11082–11092 | doi = 10.1103/PhysRevB.47.11082 | title = Structure of potassium cyanide at low temperature and high pressure determined by neutron diffraction |author1=H. T. Stokes |author2=D. L. Decker |author3=H. M. Nelson |author4=J. D. Jorgensen | pmid = 10005242 | bibcode = 1993PhRvB..4711082S | url = https://zenodo.org/record/1233725 | type = Submitted manuscript }}.</ref>

==Applications==
KCN and [[sodium cyanide]] (NaCN) are widely used in [[organic synthesis]] for the preparation of [[nitrile]]s and [[carboxylic acid]]s, 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]].{{cn|date=November 2025}}

KCN is used as a [[photographic fixer]] in the wet plate [[collodion process]].<ref>{{cite news |last1=Wood |first1=Gaby |title=Collodion photography: self-portrait in cyanide |url=https://www.telegraph.co.uk/culture/photography/7871164/Collodion-photography-self-portrait-in-cyanide.html |access-date=5 November 2025 |work=www.telegraph.co.uk |agency=The Telegraph |publisher=The Telegraph |date=6 July 2010}}</ref> 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.{{needs update|reason=It was used, but I can't find it for sale to the general public anymore, probably because of its popularity in suicides.|date=November 2025}}

{{anchor|Cyanogen soap}}
In the 19th century, cyanogen [[soap]], a preparation containing potassium cyanide, was used by [[photography|photographers]] to remove [[silver]] stains from their hands.<ref>{{ cite journal | editor1-last = Crookes | editor1-first = William | title = Photographic Notes and Queries | journal = The Photographic News: A Weekly Record of the Progress of Photography | volume = 1 | number = 1 | date = 10 September 1858 | pages = 10–12 | url = https://books.google.com/books?id=pvU_AAAAYAAJ&q=%22cyanogen+soap%22&pg=PA11 | publisher =  Cassell, Petter, and Galpin | location = London }}</ref>{{rp|p=11}}<ref>{{cite book |url=https://www.gutenberg.org/ebooks/50636 |title=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 |first1=G. Lathom |last1=Browne |first2=C. G. |last2=Stewart |location=London |publisher=Law Publishers and Booksellers |orig-date=1883}}.</ref>{{rp|p=73}}

===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:
:{{chem2|4 Au + 8 KCN + O2 + 2 H2O -> 4 K[Au(CN)2] + 4 KOH }}

=== 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 separated and analyzed in a spectro-photometer.<ref>{{Cite journal |last=Pawlowski |first=Lucjan |date=March 1994 |title=Standard methods for the examination of water and wastewater, 18th edition |url=https://doi.org/10.1016/0048-9697(94)90332-8 |journal=Science of the Total Environment |volume=142 |issue=3 |pages=227–228 |doi=10.1016/0048-9697(94)90332-8 |bibcode=1994ScTEn.142..227P |issn=0048-9697|url-access=subscription }}</ref>

==Toxicity==
{{Main|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 [[convulsion]]s. Death is caused by [[Histotoxic hypoxia#Cyanide|histotoxic hypoxia]]/[[cerebral hypoxia]]. The expected LD100 dose (human) for potassium cyanide is {{val|200|–|300|u=mg}} while the median lethal dose LD50 is estimated at {{val|140|u=mg}}.<ref>{{cite book |last1=Trestrail III |first1=John Harris |title=Criminal Poisoning - Investigational Guide for Law Enforcement, Toxicologists, Forensic Scientists, and Attorneys |date=30 April 2007 |publisher=Humana |isbn=978-1588299215 |page=119 |edition=2nd ed, 2007}}</ref>

==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 (disambiguation)|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.<ref name=Ullmann/>

==References==
{{Reflist}}

==External links==
{{Potassium compounds}}
{{Molecules detected in outer space}}
{{Cyanides}}
{{Authority control}}

[[Category:Cyanides]]
[[Category:Photographic chemicals]]
[[Category:Potassium compounds]]
[[Category:Poisons]]