SULPHOCYANIC ACID, or THIOCY ANIC ACID, a substance having the chemical formula HS.CN for the normal form. The acid form, HN.CS, has not been isolated but its ethereal salts have been recognized, and possessing powerful acid properties. The hy drogen that it contains can be replaced by bases, with the formation of a series of salts known as sulphocyanides or thiocyanates, most of which are soluble in water. Potassium sulphocyanide, KSCN, may be prepared by the direct combina tion of sulphur with cyanide of potassium, KCN. The ammonium salt (NHOSCN, is obtained (together with ammonium sulphide) when car bon disulphide is heated with an alcoholic solution of ammonia. The ammonium salt is found abundantly in gashouse liquor. The sulphocyanides of the heavy metals are usually prepared by fusing them with the sulphides of the alkaline metals, preferably potassium, by double decomposition. The free acid may be prepared by heating small quantities of mer cury sulphocyanide in a stream of dry sulphuretted hydrogen gas; but its prepa ration in this manner is more or less dangerous, as it is often attended by explosions of some violence. A dilute solution may be obtained by
distilling an excess of a sulphocyanide with di lute sulphuric acid. The pure acid is a color less liquid at ordinary temperatures, with a strong characteristic smell. By the action of a freezing mixture of snow and salt, it may be obtained in the form of crystals. When heated it decomposes into hydrocyanic acid (HCN) and persulphocyanic acid (H,C,N2S2). Dilute aqueous solutions of sulphocyanic acid are stable at ordinary temperatures, but the anhy drous acid gradually passes into the polymeric substance known as disulphocyanic acid, Disulphocyanic acid, when pure and freshly prepared, is a waxy yellow substance, nearly insoluble in water. It hardens with age, and upon the application of heat it is converted into ordinary sulphocyanic acid. Trisulphocy anic acid is not known except through its me thylic salt (MeaS2C•Ha), obtained (with methyl triocarbamide) by heating methyl sulphocy anide to 356° F.