CYANIDE, in chemistry, a salt of prussic or hydrocyanic acid, the name being more usually restricted to inorganic salts, i.e., the salts of the metals; the organic salts (or esters) being termed nitriles. The properties of cyanides are treated in the article PRUSSIC ACID; reference should also be made to the articles on the particular metals. The most important cyanide commercially is sodium cyanide, which receives application in the "cyanide process" of gold extraction (see GOLD), as an insecticide, in case-hardening, electroplating, in the separation of minerals by flotation, and in organic syntheses.
(I) Sodium Process.—Metallic sodium is melted with char coal in a steel vessel, and dry ammonia gas blown into the liquid at a dull red heat (600°C.) ; the sodium and carbon combine with the nitrogen of the ammonia, yielding sodium cyanide which is liquid at this temperature. The combination really takes place in three stages; sodamide is first formed, but it has only a momentary existence, reacting at once either with charcoal or with sodium cyanide, to form sodium cyanamide or which then takes up carbon more slowly, yielding sodium cya nide, NaCN2+C= 2NaCN. To complete this last reaction the temperature is raised finally to about 850°C. After cooling, but while still molten, the finished cyanide is filtered through iron turnings into moulds of convenient size. The product is a pure white crystalline material containing 97.5% of sodium cyanide.
(2) Formamide Process.—When the vapour of formamide is passed over a catalyst at a moderately high temperature, e.g., 600°C., it decomposes almost completely into hydrocyanic acid and water, The former is absorbed in caustic soda and the resulting solution of sodium cyanide is evap orated to dryness in a vacuum. The briquetted product contains about 90% of sodium cyanide. This process has been made eco nomically possible by the development of the high-pressure tech nique of chemical industry; the necessary formamide is obtained by the direct combination of carbon monoxide and ammonia at 8o° to 9o° under a pressure of 200 atmospheres. The reaction takes place most readily in presence of sodium methoxide, ob tained by dissolving sodium in methanol (methyl alcohol).
(3) "Schlempe" Process.—The juice of the sugar beet con tains, in addition to sugar, certain nitrogenous compounds. After the sugar has been removed as completely as possible, there re mains a dark brown syrup, known in Germany as "Schlempe," which contains a large part of these constituents. This is dried and then distilled in retorts resembling those used in coal-gas manufacture. The gas evolved contains, among other things, am monia and the three methylamines; when heated to I,000°C. the latter decompose, yielding hydrocyanic acid. On leaving the heating stoves the gases contain ammonia, hydrocyanic acid and carbon dioxide; the ammonia is recovered by treatment with sul phuric acid after which the hydrocyanic acid is washed out with water, the dilute solution produced (about 2% HCN) is distilled and the hydrocyanic acid absorbed in caustic soda. This product resembles that of the formamide process.
(4) The Cyanamide Process begins with calcium cyanamide (q.v.), which is manufactured in large quantities by the direct union of atmospheric nitrogen with calcium carbide. A mixture of calcium cyanamide, common salt, and a little calcium carbide is fed into an electric furnace, consisting of a vertical shaft with carbon bottom and a single, suspended carbon electrode. Alter nating current passes between the electrode and the charge, heat ing it to I,200°-1,400°C. The fused furnace charge is run off almost continuously on to a rotating, water-cooled, steel drum which cools it rapidly. The product is obtained in the form of thin, grey flakes and contains cyanide equivalent to 48 to so% of sodium cyanide. The formation of cyanide in this process is due to the combination of calcium cyanamide with carbon, = Ca (CN ),. A part of the necessary carbon is already present in the calcium cyanamide owing to its method of forma tion, the addition of calcium carbide to the charge furnishes a further quantity in a more chemically ac tive form. Rapid cooling of the melt is necessary to prevent the decomposition of calcium cyanide which takes place between 700° and 400°C.
(5) The Carbonate Process, historically the oldest of those in use, was first described by L. Thompson in 1838. An intimate mixture of sodium carbonate, carbon and iron is heated at 950°C. in a nichrome retort lined with steel with a current of nitrogen em ployed in excess : the iron act ing as a catalyst. This process presents many difficulties, but has nevertheless obtained a measure of success as a producer of hydrocyanic acid.
(6) Arc Process.—When a mixture of methane and nitrogen, diluted with hydrogen, is passed through a high-tension electric arc, hydrocyanic acid is formed, Control of Insect and Other Pests.—The value of hydro cyanic acid as an insecticide was first recognized in 1886 by D. W. Coquillett, who used it in the control of scale insects on citrus trees in .California. The methods of work were standardized by the investigations of R. S. Woglum in 1907-10, and the method spread to most of the fruit-growing countries of the world. Hydrocyanic acid was first used for the destruction of vermin in the sleeping carriages of the Cape Government Railway in 1898, and its use for similar purposes spread so rapidly that the town council of Johannesburg found it necessary, in 1916, to draw up by-laws for the licensing of fumigators and for regulating the practice. Hydrocyanic acid was authorized as a fumigant in the United States Quarantine Regulations of 191o, but was first used on a large scale for killing rats in ships at New Orleans in 1914.
In the fumigation of plants it is important to secure a con centration of hydrocyanic acid which will kill the insect pest with out injury to the plant ; since these are more susceptible to injury in sunlight than in the dark, fumigation is done at night and at 38°-65°F. Some plants are more easily damaged than others.
In order to confine the gas, plants growing in the open are covered with a tent of closely-woven cotton cloth, measuring usu ally from 36 to 45ft. across, which hangs down, touching the ground all round. The distance round the bottom of this tent is then measured and the distance over the top of the tree read off from a scale which is painted on the cloth itself ; the quantity of cyanide required for the enclosure is then read off from tables constructed by Woglum and based on the use of i oz. of potas sium cyanide (or 4 oz. of sodium cyanide) for 1 oo cu.ft. of en closed space ; allowances are made for the size of tree and the nature of the insect to be dealt with. The exposure to the gas is usually one hour. The hydrocyanic acid is generated by mixing together sodium cyanide (1 oz.), concentrated sulphuric acid (I-, fluid oz.) and water (2 oz.) . The water is placed in a 2 to 3gal. stoneware jug, the sulphuric acid added, the charge of cyanide is thrown in, and the tent closed at once. In more gas-tight enclosures, such as greenhouses, a smaller dosage is sufficient; in some cases a repetition of the treatment after a suitable in terval may be needful, eggs and pupae being more resistant than the larvae and mature insects; most scale insects require about 21 oz. per i,000 cubic feet.
In the fumigation of ships and mills, modifications of the pro cedure are necessary owing to the larger quantity of gas required and the difficulty of protecting the operator from danger, several hours being allowed for the gas to escape.
The necessity of handling strong sulphuric acid and difficulties in disposal of the poisonous acid residues have led to the intro duction of pure liquid hydrocyanic acid. This was first manufac tured commercially in California in 1917 and in South Africa in 192o. In California, the liquid, which contains 2 to 4% water, is supplied in iron drums holding I oo lb. each. It is applied by means of a portable vessel containing 17 to 21 lb. with attached hand pump ; each stroke of the pump delivers a measured quan tity of the liquid to a spraying nozzle which is pushed into the tent; 2 2 c.c. of the liquid acid are equivalent to 1 oz. of sodium cyanide.
Liquid hydrocyanic acid is unstable and may even explode. For this reason its use was restricted to districts in which distribu tion by .motor transport from a local factory was possible. The instability is principally due to the presence of traces of am monia gradually formed by the action of water on the acid itself. Small quantities of a strong acid neutralize the ammonia and therefore confer temporary stability; larger quantities are use less because they accelerate the formation of ammonia. Recently many other substances which neutralize ammonia have been pro posed ; among these, substances which also act as detectors are especially interesting. Owing to its faint smell the presence of a dangerous quantity of hydrocyanic acid may easily escape notice (the smell is much more perceptible when one is smoking). The addition of 2 or 3% of such violently lachrymatory substances as cyanogen chloride, ethyl chlorocarbonate or ethyl bromoacetate is said to stabilize the acid and also give unmistakable warning of its presence in the air. Stabilized, liquid hydrocyanic acid, absorbed in granular diatomite, is marketed under the name of Cyclon.
The impure calcium cyanide produced by the cyanamide process gives off hydrocyanic acid by mere contact with moist air. Where the residue of lime is harmless the powdered material may therefore be used instead of hydrocyanic acid ; for example, it has been used for killing rabbits by blowing the dust into their burrows.