HYDROCYANIC ACID, or PRUSSIC ACID, an exceedingly poisonous substance, discovered' y Scheele in 1782. It has the chem ical formula HCN, and may be formed by passing a series of electric sparks through a mixture of nitrogen and acetylene (q.v.). The reaction in this case is 2N + C,f12.2HCN. The pure acid is more conveniently prepared by passing sulphuretted hydrogen gas, H,S, over dry mercuric cyanide, Hg(CN),, hydrocyanic acid and mercuric sulphide being formed, ac cording to the equation Hg(CN),+ H,S 2HCN HgS. its thus prepared, hydrocyanic acid is a volatile liquid, at 80° F. and freezing at 5° F., and possessing a peculiar smell of bitter almonds. It is inflammable, pro ducing by its combustion carbon dioxide. It is so intensely poisonous that a single drop of the anhydrous acid produces instant death when wallowed, and its vapor, even when mixed with considerable quantities of air, is also exceed ingly poisonous. It mixes readily with water, ether and alcohol. Although it is called an acid, and forms salts that are known as 'cyanides,' it is very weak in its acid prop erties. It does not redden litmus paper, and its salts are readily decomposed by other acids. Hydrocyanic acid is formo-nitrile, and like the nitriles is hydrolysed rapidly by boiling alkalis and slowly by mineral acids. In the presence of oxygen the cyanides dissolve all metals but lead and platinum.
Hydrocyanic acid is found in nature in the glucosides of plants, typically in amygdalin, the glucoside of bitter almonds, to which it gives its characteristic odor and taste. In dhurrin, the glucoside of sorghum, it is so plentiful that cattle have been poisoned by eating the green plant too freely. The method usually employed in the manufacture of hydrocyanic acid is to run a solution of sodium cyanide in about twice its weight of water slowly into a small excess of cold 60 per cent sulphuric acid. The resulting vapor of hydrocyanic acid is almost anhydrous, and it is completely dried over cal cis= chloride. It is kept at a temperature of 85' F., and condensed to liquid form in an ice-cooled receiver. Hydrocyanic acid may be synthesized by passing dry ammonia over wood charcoal heated to 1,300° to 1,800°, At Z000° it is liable to a reverse action, the form ing hydrocyanic acid being decomposed as at forms.
the more important compounds of hydrocyanic acid are potassium cyanide and mercuric cyanide. Potassium cyanide, KCN,
may be formed by passing nitrogen gas over a white-hot mixture of charcoal and potassium carbonate, the reaction being: It is more conveniently prepared, however,, by strongly heating potassium ferrocyanide (see below), which breaks up into potassium cyanide, carbide of iron and free nitrogen. Po tassium cyanide is a white salt, soluble in water and in alcohol and exceedingly poisonous. It is much used in electroplating as a solvent for gold and silver, and also in the extraction of gold from certain ores by the process known as (cyaniding.° It melts without decomposi tion, and in the molten state it is a powerful reducing agent. For this reason it is often used as a flux in welding and in other metal lurgical operations. It is used in solution to dissolve unaffected silver salts in the tintype process in photography.
Mercuric which is also very poison ous,.is used in medicine, and is prepared by dis solving mercuric oxide in an aqueous solution of hydrocyanic acid.
Allied to hydrocyanic acid are two other acids, known respectively as 'ferrocyanic acid,* HaFez(C.Ns)., and 'ferricyanic acids HZ, (C.N.)., which are not of any importance in the arts in the free state, but whose potassium salts are much used. Potasdhun ferrocyanide, or yellow prussiate of potash, 6H2O, is prepared on a large scale by heating a mixture of nitrogenous organic matter and caustic potash, and treating the mass with freshly prepared ferrous carbonate. Crude cyanide of potassium is formed in the first in stance, and this combines with the ferrous car bonate to form the yellow prussiate of potash and potassium carbonate, from which the yel low prussiate may be obtained in large crystals, by evaporation. When pure, the yellow prus slate is not poisonous. It is used, in the arts, as a source of Prussian blue, and also for the manufacture of potassium cyanide, which is liberated when the yellow prussiate is strongly heated.
Potassium ferricyanide, or red prussiate of potash, K.Fei(CaNs)., is prepared by passing chlorine gas through a solution of the yellow prussiate, two molecules of potassium being thereby abstracted from it, with the formation of potassium chloride as a secondary product. The red prussiate is used in the manufacture of sensitive paper for making blue-prints.