Ferridcyanogen, or FERRICYANOGEN, a compound radical supposed by chemists to exist in ferricyanic acid and the ferricyanides. It cannot exist in the free state. The most important of the ferricyanides is that of potassium, also called red prussiate of potash. It is prepared by passing a stream of chlorine gas into a solution of potassium ferrocyanide till the liquid turns dark red; on evaporation the salt is obtained in crystals. The reaction is as follows:
| Potassium Ferrocyanide. |
Chlorine. | Potassium Chloride. |
Potassium Ferricyanide. |
| + | = | + |
The chief use of potassium ferricyanide is for the manufacture of Turnbull's blue, an important dye. This substance is formed when a solution of a ferricyanide is mixed with that of a ferrous salt (green vitriol, for instance); it consists of ferrous ferricyanide, . Ferric salts yield no precipitate with ferricyanides. Strong acids separate ferricyanic acid, .