Ferrocyanogen

Ferrocyanogen, a compound radical supposed by chemists to exist in ferrocyanic acid and the ferrocyanides. It cannot exist in the free state. The most important of the ferrocyanides is the potassium salt; it is prepared in the following manner. A mixture of potassium carbonate, iron filings, and animal matter, such as dried blood, horn clippings, &c., is heated to redness in iron pots. Potassium cyanide is thus formed. The mass is extracted with water, the cyanide combines with iron, forming ferrocyanide, which dissolves, and may be purified by recrystallisation. The chief use of potassium ferrocyanide is for the preparation of Prussian blue—ferric ferrocyanide (see DYEING and CALICO-PRINTING)—which is formed on the addition of a ferric salt to solution of the ferrocyanide. A ferrous salt produces a white or light-blue precipitate of ferrous ferrocyanide. By the action of dilute sulphuric acid on potassium ferrocyanide, prussic acid is produced; if strong sulphuric acid be used, the ferrocyanide is completely decomposed, yielding carbonic oxide gas, and sulphates of iron, potassium, and ammonium. Hydrochloric acid causes the separation of ferrocyanic acid, $H\_4Fe(CN)\_6$, in small white crystals insoluble in hydrochloric acid. Most of the metallic ferrocyanides are insoluble, and many have characteristic colours. Iron cannot be detected in either ferro- or ferricyanides by any of the common reactions. Potassium ferrocyanide is not poisonous. The chemistry of these compounds is not yet thoroughly understood.