Hydrogen cyanide, orH−C≡N, is a highlyvolatile toxic liquid that is produced on a large scale industrially. It is obtained byacidification of cyanide salts.
Cyanides are produced by certain insects,[5]bacteria,fungi, andalgae. It is anantifeedant in a number of plants. Cyanides are found in substantial amounts in certain seeds and fruit stones, e.g., those ofbitter almonds,apricots,apples, andpeaches.[6] Chemical compounds that can release cyanide are known as cyanogenic compounds. In plants, cyanides are usually bound tosugar molecules in the form of cyanogenicglycosides and defend the plant againstherbivores.Cassava roots (also called manioc), an importantpotato-like food grown in tropical countries (and the base from whichtapioca is made), also contain cyanogenic glycosides.[7][8]
Hydrogen cyanide is produced by the combustion orpyrolysis of certain materials under oxygen-deficient conditions. For example, it can be detected in theexhaust ofinternal combustion engines andtobacco smoke. Certainplastics, especially those derived fromacrylonitrile, release hydrogen cyanide when heated or burnt.[12]
Cyanide is basic. The pKa of hydrogen cyanide is 9.21. Thus, addition ofacids stronger than hydrogen cyanide to solutions of cyanide salts releaseshydrogen cyanide.
Cyanide is unstable in water, but the reaction is slow until about 170 °C. It undergoeshydrolysis to giveammonia andformate, which are far less toxic than cyanide:[15]
Because of the cyanide anion's highnucleophilicity, cyano groups are readily introduced into organic molecules by displacement of ahalide group (e.g., thechloride onmethyl chloride). In general, organic cyanides are called nitriles. In organic synthesis, cyanide is a C-1synthon; i.e., it can be used to lengthen a carbon chain by one, while retaining the ability to befunctionalized.[16]
The cyanide anion reacts withtransition metals to formM-CN bonds. This reaction is the basis of cyanide's toxicity.[20] The high affinities of metals for thisanion can be attributed to its negative charge, compactness, and ability to engage in π-bonding.
Among the most important cyanide coordination compounds are thepotassium ferrocyanide and the pigmentPrussian blue, which are both essentially nontoxic due to the tight binding of the cyanides to a central iron atom.[21]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 toblueprints,bluing, andcyanotypes.
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.[12] 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 mg, or exposure to airborne cyanide of 270ppm, is sufficient to cause death within minutes.[25]
Organicnitriles do not readily release cyanide ions, and so have low toxicities.
Due to toxicity considerations, the disposal of cyanide is subject to stringent regulations. Industrial cyanide effluent is typically destroyed by oxidation usingperoxysulfuric acid,hydrogen peroxide,sulfur dioxide/copper salts ("Inco process") or all three ("Combiox Process"). Use ofsodium 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 ofhydrogen cyanide.[15]
Hydroxocobalamin reacts with cyanide to formcyanocobalamin, 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.[26]
An older cyanide antidote kit included administration of three substances:amyl nitrite pearls (administered by inhalation),sodium nitrite, andsodium thiosulfate. The goal of the antidote was to generate a large pool offerric iron (Fe3+) to compete for cyanide with cytochrome a3 (so that cyanide will bind to the antidote rather than the enzyme). Thenitritesoxidizehemoglobin tomethemoglobin, which competes with cytochrome oxidase for the cyanide ion. Cyanmethemoglobin is formed and thecytochrome oxidase enzyme is restored. The major mechanism to remove the cyanide from the body is by enzymatic conversion tothiocyanate by themitochondrial enzymerhodanese. 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 forrhodanese, needed in order to produce thiocyanate.[27]
Minimum risk levels (MRLs) may not protect for delayed health effects or health effects acquired following repeated sublethal exposure, such as hypersensitivity,asthma, orbronchitis. MRLs may be revised after sufficient data accumulates.[28]
Cyanide is mainly produced for themining ofsilver andgold: It helps dissolve these metals allowing separation from the other solids. In thecyanide 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 cyanideanions to form soluble derivatives, e.g.,[Ag(CN)2]− (dicyanoargentate(I)) and[Au(CN)2]− (dicyanoaurate(I)).[15] Silver is less"noble" than gold and often occurs as the sulfide, in which case redox is not invoked (noO2 is required). Instead, a displacement reaction occurs:
The "pregnant liquor" containing these ions is separated from the solids, which are discarded to atailing pond or spent heap, the recoverable gold having been removed. The metal is recovered from the "pregnant solution" by reduction withzinc dust or byadsorption ontoactivated carbon. This process can result in environmental and health problems. A number ofenvironmental 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.[29]
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 toiron pyrite (fool's gold), wherein half of the sulfur atoms are replaced byarsenic. Gold-containing arsenopyrite ores are similarly reactive toward inorganic cyanide.[30][31]
The second major application of alkali metal cyanides (after mining) is in the production of CN-containing compounds, usually nitriles.Acyl cyanides are produced from acyl chlorides and cyanide.Cyanogen,cyanogen chloride, and the trimercyanuric chloride are derived from alkali metal cyanides.
The cyanide compoundsodium nitroprusside is used mainly inclinical chemistry to measureurineketone bodies mainly as a follow-up todiabetic patients. On occasion, it is used in emergency medical situations to produce a rapid decrease inblood pressure in humans; it is also used as avasodilator in vascular research. The cobalt in artificialvitamin B12 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. DuringWorld War I, a copper cyanide compound was briefly used byJapanese physicians for the treatment oftuberculosis andleprosy.[32]
Cyanides are illegally used to capture live fish nearcoral reefs for theaquarium and seafood markets. The practice is controversial, dangerous, and damaging but is driven by the lucrative exotic fish market.[33]
Poachers in Africa have been known to use cyanide to poison waterholes, to kill elephants for their ivory.[34]
Cyanides are used asinsecticides for fumigating ships.[38] Cyanide salts are used for killing ants,[39] and have in some places been used as rat poison[40] (the less toxic poisonarsenic is more common).[41]
Potassium ferrocyanide is used to achieve a blue color on castbronze sculptures 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.
Deliberate cyanide poisoning of humans has occurred many times throughout history.[44]Common salts such assodium 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 achemical weapon.
Cyanide is quantified bypotentiometric 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.[47]
Because of the toxicity of cyanide, many methods for detecting it have been investigated. Benzidine gives a blue coloration in the presence offerricyanide.[48]Iron(II) sulfate added to a solution of cyanide, such as the filtrate from thesodium fusion test, givesprussian blue. A solution ofpara-benzoquinone inDMSO reacts with inorganic cyanide to form a cyanophenol, which isfluorescent. Illumination with aUV light gives a green/blue glow if the test is positive.[49]
^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
^Sharpe, A. G. The Chemistry of Cyano Complexes of the Transition Metals; Academic Press: London, 1976[page needed]
^Holleman, A. F.; Wiberg, E. (2001).Inorganic Chemistry. San Diego: Academic Press.ISBN978-0-12-352651-9.
^Andrussow, Leonid (1927). "Über die schnell verlaufenden katalytischen Prozesse in strömenden Gasen und die Ammoniak-Oxydation (V)" [About the quicka catalytic processes in flowing gases and the ammonia oxidation (V)].Berichte der Deutschen Chemischen Gesellschaft (in German).60 (8):2005–2018.doi:10.1002/cber.19270600857.
^abBiller, José (2007)."163".Interface of neurology and internal medicine (illustrated ed.). Lippincott Williams & Wilkins. p. 939.ISBN978-0-7817-7906-7.
^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".
^"Sodium Cyanide".PubChem. National Center for Biotechnology Information. 2016. Retrieved2 September 2016.Cyanide and hydrogen cyanide are used in electroplating, metallurgy, organic chemicals production, photographic developing, manufacture of plastics, fumigation of ships, and some mining processes.
^"Reregistration Eligibility Decision (RED) Sodium Cyanide"(PDF).EPA.gov. 1 September 1994. p. 7.Archived(PDF) from the original on 2022-10-10. Retrieved2 September 2016.Sodium cyanide was initially registered as a pesticide on December 23, 1947, to control ants on uncultivated agricultural and non-agricultural areas.
Institut national de recherche et de sécurité (1997). "Cyanure d'hydrogène et solutions aqueuses".Fiche toxicologique n° 4, Paris: INRS, 5 pp. (PDF file,(in French))
Institut national de recherche et de sécurité (1997). "Cyanure de sodium. Cyanure de potassium".Fiche toxicologique n° 111, Paris: INRS, 6 pp. (PDF file,(in French))
