Sodium cyanide
Sodium cyanide is a compound with the formula NaCN and the structure. It is a white, water-soluble solid. Cyanide has a high affinity for metals, which leads to the high toxicity of this salt. Its main application, in gold mining, also exploits its high reactivity toward metals. It is a moderately strong base.
Production and chemical properties
Sodium cyanide is produced by treating hydrogen cyanide with sodium hydroxide: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.
The structure of solid NaCN is related to that of sodium chloride. The anions and cations are each six-coordinate. Potassium cyanide adopts a similar structure.
When treated with acid, it forms the toxic gas hydrogen cyanide:
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. 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 to produce sodium cyanate and water:
Applications
Cyanide mining
Gold cyanidation 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 for cyanide, which induces gold metal to oxidize and dissolve in the presence of air and water, producing the salt sodium dicyanoaurate :A similar process uses potassium cyanide to produce potassium dicyanoaurate.
These soluble forms of gold 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:
Electroplating
Cyanide baths 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' 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.