Calcium cyanide also known as black cyanide, is an inorganic compound with the formula Ca(CN)2. It is a white solid, although it is rarely observed in pure form. Commercial samples can be black-gray. It is the calcium salt of cyanide. It hydrolyses readily (even on moist air) to release hydrogen cyanide. Like other similar cyanides it is very toxic.
|Systematic IUPAC name
3D model (JSmol)
CompTox Dashboard (EPA)
|Molar mass||92.1128 g/mol|
|Density||1.853 (20 °C)|
|Melting point||640 °C (1,184 °F; 913 K) (decomposes)|
|Solubility||soluble in alcohol, weak acids|
|NFPA 704 (fire diamond)|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Calcium cyanide can be prepared by treating powdered calcium oxide with boiling anhydrous hydrocyanic acid in the presence of an accelerator such as ammonia or water in order to minimize the loss of the hydrocyanic acid by polymerization. It may also be prepared by reacting liquid hydrocyanic acid with calcium carbide. Alternatively calcium cyanide may be prepared by reacting hydrocyanic acid gas with quicklime (CaO) at high temperatures around 400 °C. At higher temperatures around 600 °C calcium cyanimide is formed instead. The material prepared often is contaminated with polymeric derivatives of hydrogen cyanide, hence the black color.
Calcium cyanide hydrolyzes readily to form hydrogen cyanide gas. The presence of acid accelerated evolution of hydrogen cyanide gas. It is reactive toward oxidizing agents. Calcium cyanide is also sometimes used to produce ammonium cyanide by reacting it with ammonium carbonate.
- Ca(CN) 2 + (NH 4)2CO3 → 2 NH 4CN + CaCO3
Calcium cyanide is used almost exclusively in the mining industry. It serves as an inexpensive source of cyanide in many leaching or vat operation to obtain precious metals such as gold and silver from their ores. It does this by forming coordination complexes with the metals separating them from the ores. It is distributed in either solid flake form or in liquid form. Calcium cyanide's high toxicity to touch, inhale, or ingest makes it useful as a rodenticide. For example, it has been used in the management of the population of Indian crested porcupines (Hystrix indica). Its toxicity has been similarly exploited as an insecticide. However its high toxicity makes it unfavorable in many cases and oftentimes other less damaging chemicals are used instead. It is also used in the making of hydrogen cyanide, ammonium cyanide, and ferrocyanides.
Like other cyanide salts, this compound is highly toxic and its use is strictly regulated.
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