Editing Cyanide

{{short description|Any molecule with a cyano group (C≡N)}}
{{About|the class of chemical compounds}}
{{distinguish|Nitrile}}
{{Chembox
| ImageFile = Cyanide-montage.png
| ImageAlt = Space-filling model of the cyanide anion: carbon bound to smaller nitrogen atom
| Name=
| PIN = 
| SystematicName = Nitridocarbonate(II)
| IUPACName =
| OtherNames =
|Section1 = {{Chembox Identifiers
| CASNo = 57-12-5
| UNII_Ref = {{fdacite|correct|FDA}}
| UNII = OXN4E7L11K
| PubChem = 5975
| ChEBI = 17514
| SMILES = [C-]#N
| ChemSpiderID = 5755
| InChI = 1S/CN/c1-2/q-1
| InChIKey = XFXPMWWXUTWYJX-UHFFFAOYSA-N
}}
|Section2 = {{Chembox Properties
| Formula = {{chem2|CN−}}
| C=1|N=1
| Appearance =
| Solubility =
| ConjugateAcid = [[Hydrogen cyanide]]}}
|Section3 = {{Chembox Hazards
| MainHazards = The cyanide ion {{chem2|CN−}} is one of the most poisonous chemicals. It may cause death in minutes.
| FlashPt =
| AutoignitionPt = }}
}}

In [[chemistry]], '''cyanide''' ({{ety|el|kyanos|[[Prussian blue|dark blue]]}}) is an [[inorganic]] [[chemical compound]] that contains a {{chem2|C\tN|auto=1}} [[functional group]]. This group, known as the '''cyano group''', consists of a [[carbon]] atom [[triple-bond]]ed to a [[nitrogen]] atom.<ref>{{cite journal|url=http://goldbook.iupac.org/C01486.html |title=cyanides|website=[[IUPAC Gold Book]] |date=2014 |doi=10.1351/goldbook.C01486 |doi-access=free}}</ref>

[[Salt (chemistry)|Ionic]] cyanides contain the cyanide [[anion]] {{chem2|−C\tN}}. This anion is [[Cyanide poisoning|extremely poisonous]]. Soluble cyanide [[Salt (chemistry)|salts]] such as [[sodium cyanide]] (NaCN), [[potassium cyanide]] (KCN) and [[tetraethylammonium cyanide]] ({{chem2|[(CH3CH2)4N]CN}}) are highly toxic.<ref name="CMC">{{Cite web| url=http://www.cyanidecode.org/cyanide_environmental.php| title=Environmental and Health Effects of Cyanide| publisher=International Cyanide Management Institute| year=2006| access-date=4 August 2009| archive-date=30 November 2012| archive-url=https://web.archive.org/web/20121130094124/http://www.cyanidecode.org/cyanide_environmental.php| url-status=dead}}</ref>

[[Covalent]] cyanides contain the {{chem2|\sC\tN}} group, and are usually called [[nitrile]]s if the group is linked by a [[Single bond|single]] [[covalent bond]] to [[carbon]] atom. For example, in [[acetonitrile]] {{chem2|CH3\sC\tN}}, the cyanide group is bonded to [[methyl]] {{chem2|\sCH3}}. In [[tetracyanomethane]] {{chem2|C(\sC\tN)4}}, four cyano groups are bonded to carbon. Although nitriles generally do not release cyanide ions, the [[cyanohydrin]]s do and are thus toxic. The cyano group may be covalently bonded to atoms different than carbon, e.g., in [[cyanogen azide]] {{chem2|N3\sC\tN}}, [[phosphorus tricyanide]] {{chem2|P(\sC\tN)3}} and [[trimethylsilyl cyanide]] {{chem2|(CH3)3Si\sC\tN}}.

[[Hydrogen cyanide]], or {{chem2|H\sC\tN}}, is a highly [[Volatility (chemistry)|volatile]] toxic liquid that is produced on a large scale industrially. It is obtained by [[Acid|acidification]] of cyanide salts.

==Bonding==
The cyanide ion {{chem2|−C\tN}} is [[isoelectronic]] with [[carbon monoxide]] {{chem2|-C\tO+}} and with molecular [[nitrogen]] N≡N. A triple bond exists between C and N. The negative charge is concentrated on [[carbon]] C.<ref>Greenwood, N. N.; & Earnshaw, A. (1997). Chemistry of the Elements (2nd Edn.), Oxford:Butterworth-Heinemann. {{ISBN|0-7506-3365-4}}.{{page needed|date=July 2015}}</ref><ref>G. L. Miessler and D. A. Tarr "Inorganic Chemistry" 3rd Ed, Pearson/Prentice Hall publisher, {{ISBN|0-13-035471-6}}.{{page needed|date=July 2015}}</ref>

==Occurrence==

===In nature===
[[File:Removal of cyanide poison from cassava.jpg|thumb|left|Removal of cyanide from [[cassava]] in [[Nigeria]]]]
Cyanides are produced by certain [[bacterium|bacteria]], [[fungi]], and [[algae]]. It is an [[antifeedant]] in a number of plants. Cyanides are found in substantial amounts in certain seeds and fruit stones, e.g., those of [[bitter almond]]s, [[apricot]]s, [[apple]]s, and [[peach]]es.<ref>{{Cite web|url=https://wwwn.cdc.gov/TSP/ToxFAQs/ToxFAQsLanding.aspx?id=71&tid=19 |title=ToxFAQs for Cyanide |access-date=2008-06-28 |date = July 2006|publisher=[[Agency for Toxic Substances and Disease Registry]]}}</ref> Chemical compounds that can release cyanide are known as cyanogenic compounds. In plants, cyanides are usually bound to [[sugar]] molecules in the form of cyanogenic [[glycoside]]s and defend the plant against [[herbivore]]s. [[Cassava]] roots (also called manioc), an important [[potato]]-like food grown in tropical countries (and the base from which [[tapioca]] is made), also contain cyanogenic glycosides.<ref>{{Cite journal|first=J. |last=Vetter |title=Plant cyanogenic glycosides |journal=Toxicon |year=2000 |volume=38 |pages=11–36 |doi=10.1016/S0041-0101(99)00128-2 |pmid=10669009 |issue=1|bibcode=2000Txcn...38...11V}}</ref><ref name=jones>{{Cite journal|first=D. A. |last=Jones |title= Why are so many food plants cyanogenic? |journal=[[Phytochemistry (journal)|Phytochemistry]] |year=1998 |volume=47 |pages=155–162 |doi=10.1016/S0031-9422(97)00425-1 |pmid=9431670 |issue=2|bibcode=1998PChem..47..155J}}</ref>

The [[Madagascar]] bamboo ''[[Cathariostachys madagascariensis]]'' produces cyanide as a deterrent to grazing. In response, the [[golden bamboo lemur]], which eats the bamboo, has developed a high tolerance to cyanide.

The [[hydrogenase]] enzymes contain cyanide [[ligand]]s attached to iron in their active sites. The biosynthesis of cyanide in the [[NiFe hydrogenase]]s proceeds from [[carbamoyl phosphate]], which converts to [[cystein]]yl [[thiocyanate]], the {{chem2|CN−}} donor.<ref>{{cite journal |last1=Reissmann |first1=Stefanie |last2=Hochleitner |first2=Elisabeth |last3=Wang |first3=Haofan |last4=Paschos |first4=Athanasios |last5=Lottspeich |first5=Friedrich |last6=Glass |first6=Richard S. |last7=Böck |first7=August |title=Taming of a Poison: Biosynthesis of the NiFe-Hydrogenase Cyanide Ligands |journal=Science |volume=299 |issue=5609 |pages=1067–1070 |year=2003 |pmid=12586941 |doi=10.1126/science.1080972 |bibcode=2003Sci...299.1067R |s2cid=20488694 |url=http://pdfs.semanticscholar.org/d359/5a5928df6c6209f88e105c937ccce0a05237.pdf |archive-url=https://web.archive.org/web/20201123134841/http://pdfs.semanticscholar.org/d359/5a5928df6c6209f88e105c937ccce0a05237.pdf |archive-date=2020-11-23 |url-status=live}}</ref>

===Interstellar medium===
The [[cyanide radical]] <sup>•</sup>CN has been identified in [[interstellar space]].<ref>{{Cite journal |last=Pieniazek |first=Piotr A. |author2=Bradforth, Stephen E. |author3=Krylov, Anna I. |title=Spectroscopy of the Cyano Radical in an Aqueous Environment |date=2005-12-07 |pages=4854–4865 |issue=14 |volume=110 |url=http://www-bcf.usc.edu/~krylov/pubs/pdf/jpca-110-4854.pdf |journal=The Journal of Physical Chemistry A |pmid=16599455 |doi=10.1021/jp0545952 |bibcode=2006JPCA..110.4854P |access-date=2008-08-23 |archive-url=https://web.archive.org/web/20080911131555/http://www-bcf.usc.edu/~krylov/pubs/pdf/jpca-110-4854.pdf |archive-date=2008-09-11 |url-status=dead}}</ref> [[Cyanogen]], {{chem2|(CN)2}}, is used to measure the temperature of [[Molecular cloud|interstellar gas clouds]].<ref>{{cite journal |title = Interstellar Cyanogen and the Temperature of the Cosmic Microwave Background Radiation |author1=Roth, K. C. |author2=Meyer, D. M. |author3=Hawkins, I.|author3-link=Isabel Hawkins |journal = The Astrophysical Journal |year = 1993 |volume = 413 |issue = 2 |pages = L67–L71 |doi = 10.1086/186961 |bibcode = 1993ApJ...413L..67R |url = http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1993ApJ...413L..67R&amp;data_type=PDF_HIGH&amp;whole_paper=YES&amp;type=PRINTER&amp;filetype=.pdf}}</ref>

===Pyrolysis and combustion product===
Hydrogen cyanide is produced by the combustion or [[pyrolysis]] of certain materials under oxygen-deficient conditions. For example, it can be detected in the [[exhaust gas|exhaust]] of [[internal combustion engine]]s and [[tobacco]] smoke. Certain [[plastic]]s, especially those derived from [[acrylonitrile]], release hydrogen cyanide when heated or burnt.<ref name="CDC"/>

===Organic derivatives===
{{Main|Nitriles}}
{{see also|Isocyanide}}
In [[IUPAC nomenclature of organic chemistry|IUPAC nomenclature]], [[organic compound]]s that have a {{chem2|\sC\tN}} [[functional group]] are called [[nitrile]]s.<ref>[[IUPAC Gold Book]] [http://goldbook.iupac.org/N04151.html ''nitriles'']</ref><ref>NCBI-MeSH [https://www.ncbi.nlm.nih.gov/mesh/68009570 ''Nitriles'']</ref> An example of a nitrile is [[acetonitrile]], {{chem2|CH3\sC\tN}}. Nitriles usually do not release cyanide ions. A functional group with a hydroxyl {{chem2|\sOH}} and cyanide {{chem2|\sCN}} bonded to the same carbon atom is called [[cyanohydrin]] ({{chem2|R2C(OH)CN}}). Unlike nitriles, cyanohydrins do release poisonous [[hydrogen cyanide]].

==Reactions==
===Protonation===
Cyanide is basic. The p''K''<sub>a</sub> of hydrogen cyanide is 9.21. Thus, addition of [[acids]] stronger than hydrogen cyanide to solutions of cyanide salts releases [[hydrogen cyanide]].

===Hydrolysis===
Cyanide is unstable in water, but the reaction is slow until about 170&nbsp;°C. It undergoes [[hydrolysis]] to give [[ammonia]] and [[formate]], which are far less toxic than cyanide:<ref name=Ullmann/>
:{{chem2|CN- + 2 H2O → [[formate|HCO2-]] + [[ammonia|NH3]]}}
[[Cyanide hydrolase]] is an [[enzyme]] that [[catalyzes]] this reaction.

===Alkylation===
Because of the cyanide anion's high [[nucleophile|nucleophilicity]], cyano groups are readily introduced into organic molecules by displacement of a [[halide]] group (e.g., the [[chloride]] on [[methyl chloride]]). In general, organic cyanides are called nitriles. In organic synthesis, cyanide is a C-1 [[synthon]]; i.e., it can be used to lengthen a carbon chain by one, while retaining the ability to be [[wiktionary:functionalize|functionalized]].<ref>{{Ullmann|doi=10.1002/14356007.a17_363|title=Nitriles|year=2000|last1=Pollak|first1=Peter|last2=Romeder|first2=Gérard|last3=Hagedorn|first3=Ferdinand|last4=Gelbke|first4=Heinz-Peter|isbn=3-527-30673-0}}</ref>
:{{chem2|RX + CN- → RCN + X-}}

===Redox===
The cyanide ion is a [[reducing agent|reductant]] and is [[oxidation|oxidized]] by strong [[oxidizing agent]]s such as molecular [[chlorine]] ({{chem2|Cl2}}), [[hypochlorite]] ({{chem2|ClO-}}), and [[hydrogen peroxide]] ({{chem2|H2O2}}). These oxidizers are used to destroy cyanides in [[effluent]]s from [[gold mining]].<ref name="Young_1995">Young, C. A., & Jordan, T. S. (1995, May). Cyanide remediation: current and past technologies. In: Proceedings of the 10th Annual Conference on Hazardous Waste Research (pp. 104–129). Kansas State University: Manhattan, KS. https://engg.ksu.edu/HSRC/95Proceed/young.pdf</ref><ref name="SRK">{{Cite web |title=Cyanide Destruction {{!}} SRK Consulting |author=Dmitry Yermakov |work=srk.com |date= |access-date=2 March 2021 |url= https://www.srk.com/en/publications/cyanide-destruction |language=English}}</ref><ref name="Botz">Botz Michael M. Overview of cyanide treatment methods. Elbow Creek Engineering, Inc. http://www.botz.com/MEMCyanideTreatment.pdf</ref>

===Metal complexation===
The cyanide anion reacts with [[transition metals]] to form [[Cyanometalate|M-CN bonds]]. This reaction is the basis of cyanide's toxicity.<ref>Sharpe, A. G. The Chemistry of Cyano Complexes of the Transition Metals; Academic Press: London, 1976{{page needed|date=July 2015}}</ref> The high affinities of metals for this [[anion]] can be attributed to its negative charge, compactness, and ability to engage in π-bonding.

Among the most important cyanide coordination compounds are the [[potassium ferrocyanide]] and the pigment [[Prussian blue]], which are both essentially nontoxic due to the tight binding of the cyanides to a central iron atom.<ref name=Holl>{{cite book |author1=Holleman, A. F. |author2=Wiberg, E. | title = Inorganic Chemistry | publisher = Academic Press | location = San Diego | year = 2001 | isbn = 978-0-12-352651-9}}</ref>
Prussian blue was first accidentally made around 1706, by heating substances containing iron and carbon and nitrogen, and other cyanides made subsequently (and named after it). Among its many uses, Prussian blue gives the blue color to [[blueprints]], [[bluing (fabric)|bluing]], and [[cyanotype]]s.

==Manufacture==
{{main|Hydrogen cyanide#Production and synthesis}}
The principal process used to manufacture cyanides is the [[Andrussow process]] in which gaseous [[hydrogen cyanide]] is produced from [[methane]] and [[ammonia]] in the presence of [[oxygen]] and a [[platinum]] [[catalyst]].<ref>{{cite journal
|title=Über die schnell verlaufenden katalytischen Prozesse in strömenden Gasen und die Ammoniak-Oxydation (V) |trans-title=About the quicka catalytic processes in flowing gases and the ammonia oxidation (V) |language=de |author-link1=Leonid Andrussow |first1=Leonid |last1=Andrussow |journal=Berichte der Deutschen Chemischen Gesellschaft |volume=60 |issue=8 |pages=2005–2018 |year=1927 |doi=10.1002/cber.19270600857}}</ref><ref>{{cite journal |title=Über die katalytische Oxydation von Ammoniak-Methan-Gemischen zu Blausäure |trans-title=About the catalytic oxidation of ammonia-methane mixtures to cyanide |language=de |first1=L. |last1=Andrussow |journal=[[Angewandte Chemie]] |volume=48 |issue=37 |pages=593–595 |year=1935 |doi=10.1002/ange.19350483702 |bibcode=1935AngCh..48..593A}}</ref>

:{{chem2|2 CH4 + 2 NH3 + 3 O2 → 2 HCN + 6 H2O}}

Sodium cyanide, the precursor to most cyanides, is produced by treating [[hydrogen cyanide]] with [[sodium hydroxide]]:<ref name=Ullmann />
:{{chem2|HCN + NaOH → NaCN + H2O}}

==Toxicity==
{{Main|Cyanide poisoning}}
Among the most toxic cyanides are [[hydrogen cyanide]] (HCN), [[sodium cyanide]] (NaCN), [[potassium cyanide]] (KCN), and [[calcium cyanide]] ({{chem2|Ca(CN)2}}). The cyanide anion is an [[enzyme inhibitor|inhibitor]] of the [[enzyme]] [[cytochrome c oxidase]] (also known as aa<sub>3</sub>), the fourth complex of the [[electron transport chain]] found in the [[Inner mitochondrial membrane|inner membrane]] of the [[mitochondria]] of [[Eukaryote|eukaryotic]] cells. It attaches to the iron within this protein. The binding of cyanide to this enzyme prevents transport of electrons from [[cytochrome c]] to oxygen. As a result, the electron transport chain is disrupted, meaning that the cell can no longer aerobically produce [[adenosine triphosphate|ATP]] for energy.<ref>{{cite book|last1=Nelson|first1=David L.|last2=Cox|first2=Michael M.|title=Lehniger Principles of Biochemistry|publisher=[[Worth Publishers]]|year=2000|location=New York|edition=3rd|isbn=978-1-57259-153-0|pages=[https://archive.org/details/lehningerprincip01lehn/page/668 668,670–71,676]|url=https://archive.org/details/lehningerprincip01lehn/page/668}}</ref> Tissues that depend highly on [[aerobic respiration]], such as the [[central nervous system]] and the [[heart]], are particularly affected. This is an example of [[histotoxic hypoxia]].<ref name=Biller>{{cite book
|title=Interface of neurology and internal medicine
|edition=illustrated
|first1=José
|last1=Biller
|publisher=Lippincott Williams & Wilkins
|year=2007
|isbn=978-0-7817-7906-7
|chapter=163
|page=939
|chapter-url=https://books.google.com/books?id=SRIvmTVcYBwC&pg=PA939}}
</ref>

Hydrogen cyanide, which is a gas, kills by inhalation. For this reason, working with hydrogen cyanide requires wearing an air respirator supplied by an external oxygen source.<ref name="CDC">{{Cite web|url=https://emergency.cdc.gov/agent/cyanide/basics/facts.asp|title=Facts about cyanide:Where cyanide is found and how it is used|last=Anon|date=June 27, 2013|work=CDC Emergency preparedness and response|publisher=Centers for Disease Control and Prevention|access-date=10 December 2016}}</ref> Hydrogen cyanide can be produced by adding acid to a solution containing a cyanide salt. Alkaline solutions of cyanide are safer to use because they do not evolve hydrogen cyanide gas. Oral ingestion of a small quantity of solid cyanide or a cyanide solution of as little as 200&nbsp;mg, or exposure to airborne cyanide of 270 [[parts per million|ppm]], is sufficient to cause death within minutes.<ref name=Biller/>

Organic [[nitrile]]s do not readily release cyanide ions, and so have low toxicities. 

===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>{{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>

===Antidote===
[[Hydroxocobalamin]] reacts with cyanide to form [[cyanocobalamin]], which can be safely eliminated by the kidneys. This method has the advantage of avoiding the formation of methemoglobin (see below). This antidote kit is sold under the brand name Cyanokit and was approved by the U.S. FDA in 2006.<ref>{{EMedicine|article|814287|Cyanide Toxicity|treatment}}</ref>

An older cyanide antidote kit included administration of three substances: [[amyl nitrite]] pearls (administered by inhalation), [[sodium nitrite]], and [[sodium thiosulfate]]. The goal of the antidote was to generate a large pool of [[ferric]] iron ({{chem2|Fe(3+)}}) to compete for cyanide with cytochrome a<sub>3</sub> (so that cyanide will bind to the antidote rather than the enzyme). The [[nitrite]]s [[oxidize]] [[hemoglobin]] to [[methemoglobin]], which competes with cytochrome oxidase for the cyanide ion. Cyanmethemoglobin is formed and the [[cytochrome oxidase]] enzyme is restored. The major mechanism to remove the cyanide from the body is by enzymatic conversion to [[thiocyanate]] by the [[mitochondrial]] enzyme [[rhodanese]]. Thiocyanate is a relatively non-toxic molecule and is excreted by the kidneys. To accelerate this detoxification, sodium thiosulfate is administered to provide a sulfur donor for [[rhodanese]], needed in order to produce thiocyanate.<ref>{{cite journal | last1 = Chaudhary | first1 = M. | last2 = Gupta | first2 = R. | year = 2012 | title = Cyanide Detoxifying Enzyme: Rhodanese | journal = Current Biotechnology | volume = 1 | issue = 4 | pages = 327–335 | doi = 10.2174/2211550111201040327}}</ref>

===Sensitivity===
Minimum risk levels (MRLs) may not protect for delayed health effects or health effects acquired following repeated sublethal exposure, such as hypersensitivity, [[asthma]], or [[bronchitis]]. MRLs may be revised after sufficient data accumulates.<ref>{{cite report|title=Toxicological Profile for Cyanide |publisher=U.S. Department of Health and Human Services |date=2006 |url=https://www.atsdr.cdc.gov/toxprofiles/tp8.pdf |archive-url=https://web.archive.org/web/20040331014808/http://www.atsdr.cdc.gov/toxprofiles/tp8.pdf |archive-date=2004-03-31 |url-status=live |pages=18–19}}</ref>

==Applications==

===Mining===
{{Main|Gold cyanidation}}
Cyanide is mainly produced for the [[mining]] of [[silver]] and [[gold]]: It helps dissolve these metals allowing separation from the other solids. In the ''[[cyanide process]]'', finely ground high-grade ore is mixed with the cyanide (at a ratio of about 1:500 parts NaCN to ore); low-grade ores are stacked into heaps and sprayed with a cyanide solution (at a ratio of about 1:1000 parts NaCN to ore). The precious metals are complexed by the cyanide [[anion]]s to form soluble derivatives, e.g., {{chem2|[Ag(CN)2]-}} (dicyanoargentate(I)) and {{chem2|[Au(CN)2]-}} (dicyanoaurate(I)).<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> Silver is less [[Noble metal|"noble"]] than gold and often occurs as the sulfide, in which case redox is not invoked (no {{chem2|O2}} is required). Instead, a displacement reaction occurs:
:{{chem2|Ag2S + 4 NaCN + H2O → 2 Na[Ag(CN)2] + NaSH + NaOH}}
:{{chem2|4 Au + 8 NaCN + O2 + 2 H2O → 4 Na[Au(CN)2] + 4 NaOH}}
The "pregnant liquor" containing these ions is separated from the solids, which are discarded to a [[tailing pond]] or spent heap, the recoverable gold having been removed. The metal is recovered from the "pregnant solution" by reduction with [[zinc]] dust or by [[adsorption]] onto [[activated carbon]]. This process can result in environmental and health problems. A number of [[List of gold mining disasters|environmental disasters]] have followed the overflow of tailing ponds at gold mines. Cyanide contamination of waterways has resulted in numerous cases of human and aquatic species mortality.<ref>{{cite journal |last1=Kumar |first1=Rahul |last2=Saha |first2=Shouvik |last3=Sarita |first3=Dhaka |last4=Mayur B. |first4=Kurade |last5=Kang |first5=Chan Ung |last6=Baek |first6=Seung Han |last7=Jeong |first7=Byong-Hun |title=Remediation of cyanide-contaminated environments through microbes and plants: a review of current knowledge and future perspectives |journal=Geosystem Engineering |date=2016 |volume=70 |issue=1 |pages=28–40 |doi=10.1080/12269328.2016.1218303 |s2cid=132571397 |url=https://www.tandfonline.com/doi/full/10.1080/12269328.2016.1218303 |access-date=24 April 2022}}</ref>

Aqueous cyanide is hydrolyzed rapidly, especially in sunlight. It can mobilize some heavy metals such as mercury if present. Gold can also be associated with arsenopyrite (FeAsS), which is similar to [[iron pyrite]] (fool's gold), wherein half of the sulfur atoms are replaced by [[arsenic]]. Gold-containing arsenopyrite ores are similarly reactive toward inorganic cyanide.<ref>{{Cite journal |last1=Konyratbekova |first1=Saltanat Sabitovna |last2=Baikonurova |first2=Aliya |last3=Akcil |first3=Ata |date=2015-05-04 |title=Non-cyanide Leaching Processes in Gold Hydrometallurgy and Iodine-Iodide Applications: A Review |url=http://www.tandfonline.com/doi/abs/10.1080/08827508.2014.942813 |journal=Mineral Processing and Extractive Metallurgy Review |language=en |volume=36 |issue=3 |pages=198–212 |doi=10.1080/08827508.2014.942813 |bibcode=2015MPEMR..36..198K |issn=0882-7508}}</ref><ref>{{Cite journal |last1=Zhang |first1=Yan |last2=Cui |first2=Mingyao |last3=Wang |first3=Jianguo |last4=Liu |first4=Xiaoliang |last5=Lyu |first5=Xianjun |date=2022 |title=A review of gold extraction using alternatives to cyanide: Focus on current status and future prospects of the novel eco-friendly synthetic gold lixiviants |url=https://linkinghub.elsevier.com/retrieve/pii/S0892687521005653 |journal=Minerals Engineering |language=en |volume=176 |pages=107336 |doi=10.1016/j.mineng.2021.107336|bibcode=2022MiEng.17607336Z}}</ref>

===Industrial organic chemistry===
The second major application of alkali metal cyanides (after mining) is in the production of CN-containing compounds, usually nitriles. [[Acyl cyanide]]s are produced from acyl chlorides and cyanide. [[Cyanogen]], [[cyanogen chloride]], and the trimer [[cyanuric chloride]] are derived from alkali metal cyanides.

===Medical uses===
The cyanide compound [[sodium nitroprusside]] is used mainly in [[clinical chemistry]] to measure [[urine]] [[ketone bodies]] mainly as a follow-up to [[diabetic]] patients. On occasion, it is used in emergency medical situations to produce a rapid decrease in [[blood pressure]] in humans; it is also used as a [[vasodilator]] in vascular research. The cobalt in artificial [[Vitamin B12|vitamin B<sub>12</sub>]] contains a cyanide ligand as an artifact of the purification process; this must be removed by the body before the vitamin molecule can be activated for biochemical use. During [[World War I]], a copper cyanide compound was briefly used by [[Japan]]ese physicians for the treatment of [[tuberculosis]] and [[leprosy]].<ref>{{Cite journal|last=Takano |first=R. |date=August 1916 |title=The treatment of leprosy with cyanocuprol |journal=The Journal of Experimental Medicine |volume=24 |issue= 2|pages=207–211 |url=http://www.jem.org/cgi/content/abstract/24/2/207 |access-date=2008-06-28 |doi=10.1084/jem.24.2.207 |pmc=2125457 |pmid=19868035}}</ref>

===Illegal fishing and poaching===
{{Main|Cyanide fishing}}
Cyanides are illegally used to capture live fish near [[coral reef]]s for the [[aquarium]] and seafood markets. The practice is controversial, dangerous, and damaging but is driven by the lucrative exotic fish market.<ref name="crc">Dzombak, David A; Ghosh, Rajat S; Wong-Chong, George M. ''Cyanide in Water and Soil''. [[CRC Press]], 2006, Chapter 11.2: "Use of Cyanide for Capturing Live Reef Fish".</ref>

Poachers in Africa have been known to use cyanide to poison waterholes, to kill elephants for their ivory.<ref>[http://www.abc.net.au/news/2013-09-25/zimbabwe-poachers-kill-80-elephants-with-cyanide/4981372 Poachers kill 80 elephants with cyanide in Zimbabwe] ''ABC News'', 25 September 2013. Retrieved 30 October 2015.</ref>

===Pest control===
[[M44 (cyanide device)|M44 cyanide devices]] are used in the United States to kill [[coyote]]s and other canids.<ref>{{cite journal|doi=10.1002/wsb.361|title=Animal attendance at M-44 sodium cyanide ejector sites for coyotes|journal=Wildlife Society Bulletin|volume=38|pages=217–220|year=2014|last1=Shivik|first1=John A.|last2=Mastro|first2=Lauren|last3=Young|first3=Julie K. |issue=1 |bibcode=2014WSBu...38..217S |url=http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=2419&context=icwdm_usdanwrc}}</ref> Cyanide is also used for pest control in [[New Zealand]], particularly for [[Common Brushtail Possum in New Zealand|possums]], an [[Invasive species in New Zealand|introduced marsupial that threatens the conservation of native species]] and spreads [[tuberculosis]] amongst cattle. Possums can become bait shy but the use of pellets containing the cyanide reduces bait shyness. Cyanide has been known to kill native birds, including the endangered [[Kiwi (bird)|kiwi]].<ref>{{cite web |last = Green| first = Wren |title =The use of 1080 for pest control |publisher = New Zealand Department of Conservation |date = July 2004 |url =http://www.doc.govt.nz/upload/documents/conservation/threats-and-impacts/animal-pests/use-of-1080-04.pdf| access-date = 8 June 2011}}</ref> Cyanide is also effective for controlling the [[dama wallaby]], another introduced marsupial pest in New Zealand.<ref>{{cite journal|last=Shapiro|first=Lee|date=21 March 2011|title=Effectiveness of cyanide pellets for control of dama wallabies (Macropus eugenii)|journal=New Zealand Journal of Ecology |volume=35 |issue=3 |url=http://newzealandecology.org/nzje/new_issues/NZJEcol35_3_287.pdf |archive-url=https://web.archive.org/web/20150203010818/http://newzealandecology.org/nzje/new_issues/NZJEcol35_3_287.pdf |archive-date=2015-02-03 |url-status=live |display-authors=etal}}</ref> A licence is required to store, handle and use cyanide in New Zealand.

Cyanides are used as [[insecticide]]s for fumigating ships.<ref>{{cite web|title=Sodium Cyanide|url=https://pubchem.ncbi.nlm.nih.gov/compound/sodium_cyanide|website=PubChem|publisher=National Center for Biotechnology Information|access-date=2 September 2016|date=2016|quote=Cyanide and hydrogen cyanide are used in electroplating, metallurgy, organic chemicals production, photographic developing, manufacture of plastics, fumigation of ships, and some mining processes.}}</ref> Cyanide salts are used for killing ants,<ref name="EPAReg1994">{{cite web|title=Reregistration Eligibility Decision (RED) Sodium Cyanide|url=https://archive.epa.gov/pesticides/reregistration/web/pdf/3086.pdf |archive-url=https://ghostarchive.org/archive/20221010/https://archive.epa.gov/pesticides/reregistration/web/pdf/3086.pdf |archive-date=2022-10-10 |url-status=live|website=EPA.gov|access-date=2 September 2016|page=7|date=1 September 1994|quote=Sodium cyanide was initially registered as a pesticide on December 23, 1947, to control ants on uncultivated agricultural and non-agricultural areas.}}</ref> and have in some places been used as rat poison<ref name="TariffInfo1921">{{cite web|title=Tariff Information, 1921: Hearings on General Tariff Revision Before the Committee on Ways and Means, House of Representatives|url=http://www.abebooks.com/servlet/SearchResults?tn=Tariff+Information,+1921|website=AbeBooks.com|publisher=US Congress, House Committee on Ways and Means, US Government Printing Office|access-date=2 September 2016|page=3987|date=1921|quote=Another field in which cyanide is used in growing quantity is the eradication of rats and other vermin – especially in the fight against typhus.}}</ref> (the less toxic poison [[arsenic]] is more common).<ref name="PlanetDeadly2013">{{cite web|title=Deadliest Poisons Used by Man|url=http://www.planetdeadly.com/human/deadliest-poisons-man|website=PlanetDeadly.com|access-date=2 September 2016|archive-url=https://web.archive.org/web/20160511033535/http://www.planetdeadly.com/human/deadliest-poisons-man|archive-date=11 May 2016|date=18 November 2013}}</ref>

===Niche uses===
[[Potassium ferrocyanide]] is used to achieve a blue color on cast [[bronze sculpture]]s during the final finishing stage of the sculpture. On its own, it will produce a very dark shade of blue and is often mixed with other chemicals to achieve the desired tint and hue. It is applied using a torch and paint brush while wearing the standard safety equipment used for any patina application: rubber gloves, safety glasses, and a respirator. The actual amount of cyanide in the mixture varies according to the recipes used by each foundry.

Cyanide is also used in [[jewelry]]-making and certain kinds of [[photography]] such as [[sepia toning]].

Although usually thought to be toxic, cyanide and cyanohydrins increase germination in various plant species.<ref>{{Cite journal|doi=10.1104/pp.52.1.23 |last1=Taylorson |first1=R. |last2=Hendricks |year=1973 |first2=SB |title=Promotion of Seed Germination by Cyanide |journal=Plant Physiol. |volume=52 |issue=1 |pages=23–27 |pmid=16658492 |pmc=366431}}</ref><ref>{{Cite journal|last1=Mullick |first1=P. |year=1967 |last2=Chatterji |first2=U. N. |title=Effect of sodium cyanide on germination of two leguminous seeds |journal=Plant Systematics and Evolution |volume=114 |issue=1 |pages=88–91|doi=10.1007/BF01373937|bibcode=1967PSyEv.114...88M |s2cid=2533762}}</ref>

====Human poisoning====
{{main|Cyanide poisoning}}
Deliberate cyanide poisoning of humans has occurred many times throughout history.<ref>{{Cite book
|title=Medical Management of Chemical Casualties Handbook
|edition=4th
|last1=Bernan
|publisher=Government Printing Off
|year=2008
|isbn=978-0-16-081320-7
|page=41
|url=https://books.google.com/books?id=oiw2ZzsBvsoC}}, [https://books.google.com/books?id=oiw2ZzsBvsoC&pg=PA41 Extract p. 41]
</ref>
Common salts such as [[sodium cyanide]] are involatile but water-soluble, so are poisonous by ingestion. [[Hydrogen cyanide]] is a gas, making it more indiscriminately dangerous, however it is lighter than air and rapidly disperses up into the atmosphere, which makes it ineffective as a [[chemical weapon]].

====Food additive====
Because of the high stability of their complexation with [[iron]], ferrocyanides ([[Sodium ferrocyanide]] E535, [[Potassium ferrocyanide]] E536, and Calcium ferrocyanide E538<ref>{{cite book
|title=Benders' dictionary of nutrition and food technology
|edition=7th
|first1=David A.
|last1=Bender
|first2=Arnold Eric
|last2=Bender
|publisher=Woodhead Publishing
|year=1997
|isbn=978-1-85573-475-3
|page=459
|url=https://books.google.com/books?id=IrYfDEl7XPYC}} [https://books.google.com/books?id=IrYfDEl7XPYC&pg=PA459 Extract of page 459]
</ref>) do not decompose to lethal levels in the human body and are used in the food industry as, e.g., an [[anticaking agent]] in [[table salt]].<ref>{{cite book
|title=Geochemical processes in soil and groundwater: measurement – modelling – upscaling
|first1=Horst D.
|last1=Schulz
|first2=Astrid
|last2=Hadeler
|author3=Deutsche Forschungsgemeinschaft
|publisher=Wiley-VCH
|year=2003
|isbn=978-3-527-27766-7
|page=67
|doi=10.1002/9783527609703
|url=http://onlinelibrary.wiley.com/doi/10.1002/9783527609703}}
</ref>

==Chemical tests for cyanide==
Cyanide is quantified by [[potentiometric titration]], a method widely used in gold mining. It can also be determined by titration with silver ion. 
Some analyses begin with an air-purge of an acidified boiling solution, sweeping the vapors into a basic absorber solution. The cyanide salt absorbed in the basic solution is then analyzed.<ref>{{Ullmann|doi=10.1002/14356007.a08_159.pub2|title=Cyano Compounds, Inorganic|year=2004|last1=Gail|first1=Ernst|last2=Gos|first2=Stephen|last3=Kulzer|first3=Rupprecht|last4=Lorösch|first4=Jürgen|last5=Rubo|first5=Andreas|last6=Sauer|first6=Manfred}}</ref>

===Qualitative tests===
Because of the notorious toxicity of cyanide, many methods have been investigated. Benzidine gives a blue coloration in the presence of [[ferricyanide]].<ref>{{Ullmann |author1=Schwenecke, H. |author2=Mayer, D. | title = Benzidine and Benzidine Derivatives | year = 2005 | doi = 10.1002/14356007.a03_539}}</ref> [[Iron(II) sulfate]] added to a solution of cyanide, such as the filtrate from the [[sodium fusion test]], gives [[prussian blue]]. A solution of [[1,4-Benzoquinone|''para''-benzoquinone]] in [[dimethyl sulfoxide|DMSO]] reacts with inorganic cyanide to form a cyano[[phenol]], which is [[fluorescent]]. Illumination with a [[UV light]] gives a green/blue glow if the test is positive.<ref>{{Cite journal| doi = 10.1016/0041-008X(80)90225-2 |pmid = 7423496 |title = Fluorometric determination of cyanide in biological fluids with p-benzoquinone*1 |first4 = JL |last4 = Way |first3 = RL |last3 = Morgan |first2 = GE |year = 1980 |last2 = Isom |last1 = Ganjeloo |first1 = A |journal = [[Toxicology and Applied Pharmacology]] |volume = 55 |issue = 1 |pages = 103–107 |bibcode = 1980ToxAP..55..103G}}</ref>

==References==
{{Reflist}}

==External links==
{{EB1911 Poster|Cyanide}}
{{Commons category|Cyanides}}
*[https://web.archive.org/web/20100528070140/http://www.atsdr.cdc.gov/MMG/MMG.asp?id=1073&tid=19 ATSDR medical management guidelines for cyanide poisoning (US)]
*[http://www.hse.gov.uk/pubns/firindex.htm HSE recommendations for first aid treatment of cyanide poisoning (UK)]
*[http://www.inchem.org/documents/cicads/cicads/cicad61.htm Hydrogen cyanide and cyanides] ([[CICAD]] 61)
*[http://www.inchem.org/documents/antidote/antidote/ant02.htm#SubSectionNumber:1.13.1 IPCS/CEC Evaluation of antidotes for poisoning by cyanides]
*[https://web.archive.org/web/20060517035532/http://www.npi.gov.au/database/substance-info/profiles/29.html National Pollutant Inventory – Cyanide compounds fact sheet]
*[http://www.snopes.com/food/warnings/apples.asp#add Eating apple seeds is safe despite the small amount of cyanide]
*[http://www.atsdr.cdc.gov/toxprofiles/tp8.pdf Toxicological Profile for Cyanide, U.S. Department of Health and Human Services, July 2006]

;Safety data (French)
* Institut national de recherche et de sécurité (1997). "[https://web.archive.org/web/20060220084315/http://www.inrs.fr/inrs-pub/inrs01.nsf/inrs01_ftox_view/860430FE710FCFD7C1256CE8004F67CB/$File/ft4.pdf Cyanure d'hydrogène et solutions aqueuses]". ''Fiche toxicologique n° 4'', Paris: INRS, 5 pp. (PDF file, {{in lang|fr}})
* Institut national de recherche et de sécurité (1997). "[https://web.archive.org/web/20060220084330/http://www.inrs.fr/inrs-pub/inrs01.nsf/inrs01_ftox_view/48145297F4EF18BBC1256CE8005A9FC2/$File/ft111.pdf Cyanure de sodium. Cyanure de potassium]". ''Fiche toxicologique n° 111'', Paris: INRS, 6 pp. (PDF file, {{in lang|fr}})

{{Cyanides}}
{{Inorganic compounds of carbon}}
{{Nitrogen compounds}}
{{Rodenticides}}
{{Consumer Food Safety}}
{{Authority control}}

[[Category:Cyanides]]
[[Category:Anions]]
[[Category:Blood agents]]
[[Category:Mitochondrial toxins]]
[[Category:Nitrogen(−III) compounds]]
[[Category:Toxicology]]