hydrogen cyanide, HCN, colorless, volatile, and extremely poisonous chemical compound whose vapors have a bitter almond odor. It melts at -14°C; and boils at 26°C;. It is miscible in all proportions with water or ethanol and is soluble in ether. Its water solution is a weak acid (see acids and bases) commonly known as hydrocyanic acid or prussic acid. Its salts are called cyanides. Hydrogen cyanide may be synthesized directly from ammonia and carbon monoxide or from ammonia, oxygen (or air), and natural gas. It is a byproduct of the production of coke from coal and is recovered (along with hydrogen sulfide) from coke-oven exhaust gases. It may also be prepared by reacting a cyanide salt, e.g., calcium cyanide, with a strong acid, e.g., sulfuric acid, or by thermal decomposition of formamide. Because impure hydrogen cyanide can undergo spontaneous explosive polymerization and decomposition, a small amount of stabilizer (usually phosphoric acid) is added to it. The principal use of hydrogen cyanide is in the manufacture of organic chemicals, e.g., acrylonitrile, methyl methacrylate, and adiponitrile, that are used in producing synthetic fibers and plastics. It is also used in the chemical laboratory, and is sometimes used in agriculture as a fumigant. Hydrogen cyanide is found in nature in some vegetable substances, e.g., bitter almond, peach stones, cherry and cherry laurel leaves, and sorghum; it is usually combined in glycoside molecules (see sugar) and is released when they are broken down by enzymes during metabolism.

cyanide process or cyanidation, method for extracting gold from its ore. The ore is first finely ground and may be concentrated by flotation; if it contains certain impurities, it may be roasted. It is then mixed with a dilute solution of sodium cyanide (or potassium or calcium cyanide) while air is bubbled through it. The gold is oxidized and forms the soluble aurocyanide complex ion, Au(CN)₂^-1. (Silver, usually present as an impurity, forms a similar soluble ion.) The solution is separated from the ore by methods such as filtration, and the gold is precipitated by adding powdered zinc. The precipitate usually contains silver, which is also precipitated, and unreacted zinc. The precipitate is further refined, e.g., by smelting to remove the zinc and by treating with nitric acid to dissolve the silver. The cyanide process was developed (1887) by J. S. MacArthur and others in Glasgow, Scotland. It is now the most important and widely used process for extracting gold from ores.

cyanide, chemical compound containing the cyano group, -CN. Cyanides are salts or esters of hydrogen cyanide (hydrocyanic acid, HCN) formed by replacing the hydrogen with a metal (e.g., sodium or potassium) or a radical (e.g., ammonium or ethyl). The most common and widely used cyanides are those of sodium and potassium; they are often referred to simply as "cyanide." Both are white, crystalline, chemically active compounds. They are used as insecticides, in making pigments, in metallurgy (e.g., electroplating and case hardening), and in refining gold and silver by the cyanide process. Organic cyanides are called nitriles. The ethyl ester of hydrogen cyanide (CH₃CH₂CN) is called variously ethyl cyanide, propionitrile, propane nitrile, nitrilopropane, and cyanoethane; propane nitrile is the approved name in the nomenclature system for organic chemistry adopted by the International Union of Pure and Applied Chemistry (IUPAC). Most cyanides are deadly poisons that cause respiratory failure. Symptoms of cyanide poisoning include an odor of bitter almond on the breath, dizziness, convulsions, collapse, and, often, froth on the mouth. In case of cyanide poisoning a doctor should be summoned immediately. If the poison was swallowed, vomiting should be induced. Artificial respiration should be used if needed.

or MacArthur-Forrest process

Method of extracting silver and gold from their ores by dissolving them in a dilute solution of sodium cyanide or potassium cyanide. The process—invented in 1887 by the Scottish chemists John S. MacArthur, Robert W. Forrest, and William Forrest—includes contacting the finely ground ore with the cyanide solution, separating unwanted solids from the clear solution, and recovering the precious metals from the solution by precipitation with zinc dust.

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Any chemical compound containing the combining group CN. Ionic (see ion; ionic bond) and organic cyanide compounds differ in chemical properties, but both are toxic, especially the ionic ones. Cyanide poisoning inhibits cells' oxidative (see oxidation-reduction) processes; its action is extremely rapid, and an antidote must be given promptly. Cyanides occur naturally in certain seeds (e.g., apple seeds, wild cherry pits). Cyanides, including hydrogen cyanide (HCN, or hydrocyanic acid), are used industrially in the production of acrylic fibres, synthetic rubbers, and plastics as well as in electroplating, case-hardening of iron and steel, fumigation, and concentration of ores.

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