Sodium cyanide
Toxic chemical compound (NaCN)
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Through Chemical formula, Cyanide and Solubility
Production and chemical properties
Sodium cyanide is produced by treating hydrogen cyanide with sodium hydroxide:
- HCN + NaOH → NaCN + H2O
Worldwide production was estimated at 500,000 tons in the year 2006. Formerly it was prepared by the Castner process involving the reaction of sodium amide with carbon at elevated temperatures.
- NaNH2 + C → NaCN + H2
The structure of solid NaCN is related to that of sodium chloride. The anions and cations are each six-coordinate. Potassium cyanide (KCN) adopts a similar structure.
When treated with acid, it forms the toxic gas hydrogen cyanide:
- NaCN + H+ → HCN + Na+
Because the salt is derived from a weak acid, sodium cyanide readily reverts to HCN by hydrolysis; the moist solid emits small amounts of hydrogen cyanide, which is thought to smell like bitter almonds (not everyone can smell it—the ability thereof is due to a genetic trait). Sodium cyanide reacts rapidly with strong acids to release hydrogen cyanide. This dangerous process represents a significant risk associated with cyanide salts. It is detoxified most efficiently with hydrogen peroxide (H2O2) to produce sodium cyanate (NaOCN) and water:
- NaCN + H2O2 → NaOCN + H2O
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Applications
Cyanide mining
Gold cyanidation (also known as the cyanide process) is the dominant technique for extracting gold, much of which is obtained from low-grade ore. More than 70% of cyanide consumption globally is used for this purpose. The application exploits the high affinity of gold(I) for cyanide, which induces gold metal to oxidize and dissolve in the presence of air (oxygen) and water, producing the salt sodium dicyanoaurate (or sodium gold cyanide) (NaAu(CN)2):
- 4 Au + 8 NaCN + O2 + 2 H2O → 4 Na[Au(CN)2] + 4 NaOH
A similar process uses potassium cyanide (KCN, a close relative of sodium cyanide) to produce potassium dicyanoaurate (KAu(CN)2).
These soluble forms of gold (and silver) can be converted back to the metals by electrolysis.
Chemical feedstock
Some commercially significant chemicals are derived from cyanide: cyanuric chloride, cyanogen chloride, and cyanogen.
Several specialty nitriles may be produced by alkylation of cyanide, a strong nucleophile. Illustrative is the synthesis of benzyl cyanide by the reaction of benzyl chloride and sodium cyanide:
- C6H5CH2Cl + NaCN → C6H5CH2CN + NaCl
Electroplating
Cyanide baths (solutions) of metal ions are used to electroplate many metals, including platinum, gold, and silver. Cadmium plating gives excellent corrosion resistance and is achieved using cadmium oxide and sodium cyanide. Copper 'strikes' (thin high quality layers) are traditionally formed using cyanide electroplating, these thin interstitial layers allow adhesion between metals that are usually incompatible, such as chromium and aluminium.
Niche uses
Being highly toxic, sodium cyanide is used to kill or stun rapidly such as in collecting jars used by entomologists and in widely illegal cyanide fishing.
It was used as an insecticide, rodenticide and antibacterial, but these uses were cancelled by the EPA in 1987.
Toxicity
Sodium cyanide, like other soluble cyanide salts, is among the most rapidly acting of all known poisons. NaCN is a potent inhibitor of cellular respiration, acting on mitochondrial cytochrome oxidase and hence blocking electron transport. This results in decreased oxidative metabolism and oxygen utilization. Lactic acidosis then occurs as a consequence of anaerobic metabolism. An oral dosage as small as 200–300 mg can be fatal.
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.