Prep. This salt is always formed when carbonate of potash and animal matters, as hoofs, horns, etc, are heated to redness along with iron, as in an iron pot, or with iron nails; from the incinerated mass, when cool, the salt can be dissolved out, and crystallized from the filtered solution.
Prop. & Comp. Ferrocyanide of potassium forms large yellow transparent, rhombic octahedrons, with truncated apices, having a saline and sweetish bitter taste; soluble in water; the solution is not altered either by alkalies or tincture of galls; it gives a precipitate with sulphate of iron, which is at first white, but soon changes to blue; with sulphate of copper a chocolate brown or maroon red, and with sulphate of zinc a white precipitate. By heat ferrocyanide of potassium first loses 12.6 per cent. of water, and becomes white; and is afterwards decomposed, leaving an ash soluble in hydrochloric acid, and precipitated by ammonia. This precipitate, consisting of sesquioxide of iron, amounts to 18.7 percent. of the salt. When heated with dilute sulphuric acid, an odour of hydrocyanic acid is evolved. Composition (2 K Cy + Fe Cy + 3 HO), or a double cyanide of potassium and iron. In the Pharmacopoeia its formula is thus represented, K2 Fe Cy3 + 3 HO. Cyanogen (Cy) = C2 N.
Use. It is employed in the preparation of hydrocyanic acid, and not used medicinally; although represented above as a double cyanide, the grouping of the elements is probably not in that form; for the salt is by no means poisonous even in large doses; there are also chemical, as well as therapeutic, reasons in favour of its containing a peculiar radical. A watery solution of the ferrocy-anide of potassium is used for testing.
Prop. & Comp. K3 Fe2 Cy6. In prismatic crystals of a fine red colour; soluble in water. The solution gives no precipitate with the persulphate of iron, but a dark blue with the protosalts of this metal. Introduced as a test into the Pharmacopoeia to distinguish between the proto- and per-salts of iron.
[Potassii Cyanidum. Cyanide of Potassium. U. S. Made by heating together Ferrocyanide of Potassium and Carbonate of Potassa. The newly formed cyanide, while melted, is poured carefully off the precipitated oxide of iron. It occurs in white opaque, amorphous masses, having the smell and taste of hydrocyanic acid. Its medicinal properties are identical with those of hydrocyanic acid, and it has the advantage of being a much more stable compound. Dose, 1/8 of a grain, which may be gradually and carefully increased to 1/2 a grain.]
Sapo Mollis. Soft Soap. [Not officinal in U. S. P.] A compound containing potash. It is described under Olive Oil.