A Manual of the Chemistry of the Carbon Compounds

THE CARBON COMPOUNDS. 55
mixing the alcoholic solution with ether, the compound is precipitated
as a crystalline powder. When exposed to the air, it absorbs
oxygen; prussic acid is given off, and prhssian blue formed; this
change takes place more quickly on the application of heat Hydrir
ferrocyanide is a strong acid which decomposes not only carbonates
but also acetates and oxaktes. On boiling its aqueous solution hydro
cyanic acid is given off and a white precipitate of ferrous ferrocyanidi 1
FeaCfy = Fe3(CN)fl is formed :—
3H4(CN)0 « 12HCN + Fe8(CNj6
This decomposition occurs in the preparation of prussic acid from
potassium ferrocyanide and dilnte sulphuric acid.
The ferrocyamdes of other metals are either sparingly soluble in
water or insoluble, and are therefore obtained as precipitates by adding
a solution of potassium ferrocyanide to solutions of metallic salts;
potassium ferrocyanide is for this reason used as a reagent-. Tints
with solutions of barium salts it gives a crystalline precipitate having
the composition BaK2Cfy, and by adding a solution of the ferrocyaoide
to a solution of copper sulphate, a red precipitate of copper
ferrocyanide Cu1|SOfy 19 formed; whilst by adding a solution of a
cupric salt to, an excess of potassium ferrocyanide a brown precipitate
CuKaCfy is obtained.
Potassium Ferricyanidc K3Cfdy.—This compound, commonly
known as red prussiate of potash, is prepared by passing chlorine int©
a solution of potassium ferrocynnide:—
K4(CN)8Fe + Cl = KC1 + K3(CN)4Fo
Potassium chloride and ferricyauide are formed and separated fromone
another by crystallization. The red prussiate crystallizes in large,
dark red, mouoclinic prisms, solnble in water with a brownish green
colour. By reducing agents it is re-converted into a ferrocyanide;
this is easily effected by adding potassium amalgam to its aqueous
solution. Its alkaline solution is, therefore, a powerful oxidizing
agent; it converts chromic oxide into chromic acid, and manganese
monoxide into the dioxide:—
6TCg(CN)0Fe + 10HOK + Cr8O3 = 6K4(CN)6Fe + 2K2Cr0, +• 5HjO
Ferric salts are not precipitated by the addition of potassium ferrieyauide,
the solution only assumes a dark brown colour ; but with
ferrons salts it produces a dark bhte precipitate, called Turnlults
bhte, which is generally considered to consist of ferrous ferricijanidi
Fes(Cfdy)2. This precipitate, however, has the same composition.
and the same properties as the compound %|N [ Cfya, \vhich, aa we-
have seen, forms the chief constituent of prussiatt blue, and it appears
highly probable that the two compounds are identical. Its formation
is easily explained. By bringing together ferricyanide of potassium
and a ferrous salt, a part of the latter is oxidized, a ferric salt being