A Manual of the Chemistry of the Carbon Compounds

56 TEE CHEMISTRY OF
formed and a ferrocyanide, from which the mixture of the two iron
salts then precipitates prussian blue:—
2K3(CN)0Fe + SFeCl,, + HjO « 2HK3(CN)0Fe + FeO + Fe2Cl0 =
Hydric Fmiicyanidc, Hydroferricyanic Add, HsCfdy.—To prepare
this acid a solution of red prussiate of potash is precipitated with
acetate of lead, and the precipitate, after washing, decomposed hy
dilute sulphuric acid. On evaporating the filtrate, the acid crystallizes
in brown needles, which have a sour and astringent taste.
Exposed to the air they give off prussic acid, and assume a blue
colour.
NITROPRUSSIDES.
By adding a solution of potassium nitrite to the precipitate, produced
by adding potassium cyanide to a solution of ferrous sulphate,
jwlassium nitropnusich is formed, according to the following equation
:—
KFe2Cy6 + KNO8 = KjFeCy^O + FeO, "
An excess of the nitrite at the same time oxidizes the ferrous oxide
to ferrio oxide. The nitroprussides are distinguished by their fine
red colour; the sodium salt, which crystallizes the best of all, is generally
prepared from potassium ferrocyanide, by heating it with dilute
nitric acid, until the solution no longer gives with ferrous sulphate a
blue but a slate-coloured precipitate. During this reaction gases are
evolved, containing nitrogen, carbon dioxide, cyanogen, and hydrocyanic
acid. On cooling the solution, potassium nitrate crystallizes
out, the mother liquor is neutralized with sodium carbonate, the solution
concentrated by evaporation, and the nitroprusside separated from
potassium and sodium nitrate by crystallization.
Sodium Nitropnisside NaJ?eCy6NO forms ruby-red crystals. Its
aqueous solution gives with alkaline sulphidea an intense purple
coloration, which soon disappears; it is therefore used as a reagent
for the detection of soluble sulphides; free hydrogen sulphide ia notcoloured
hy it.
PLATINOCYANIDES.
By suspending a mixture of barium carbonate and platinum
dichloride in boiling water, and passing hydrocyanic acid throxigh
the liquid, barium platinoeyanidc is formed, which, on cooling,
crystallizes in large mouoclinic prisms, Pt(CN)4Ba + 4H2O. This
salt, like all other soluble platinocyauides, is remarkable for the
beautiful colours which it exhibits, the crystals appearing greeu
when viewed along the principal axis, sulphur-yellow at right angles
to it, and in reflected light the faces of the crystals exhibit a blue