A Manual of the Chemistry of the Carbon Compounds

52 THE CEEMI8TRY OF
The commeroial product is prepared by fusing the ferrocyanide
with potassium carbonate:—
Fe(CN)0K4 + K2COS - 5KCN + KOCN + Fe + CO,
On heating the mixture, carbon dioxide escapes, and the iron
separates in the form of a spongy mass, from which the fused cyanide
is poured off. The salt thus obtained is mixed with some cyanato,
the presence of which, however, is of little or no consequence for
moat of the purposes for which potassium cyanide is used.
Potassium cyanide is largely used as a chemical reagent, in galvanic
silvering and gilding, and in photography, &c, It is very
soluble in water, sparingly in cold, more so in not alcohol. At a red
heat it fuses to a clear liquid, which slowly takes up oxygen from the
air. This takes place more quickly in presence of metallic oxides,
the cyanide being converted into the cyanate; on this account it is
used as a reducing agent in blow-pipe experiments. On fusing it
with sulphur, potassium sulphocyanide is formed, KSCN. Potassium
cyanide is decomposed by the weakest acids, even by moist carbon dioxide
; it therefore smells of prussic acid, and is as dangerous a poison.
Sodium Cyanide NaCN crystallizes likewise in cubes, and bas the
greatest resemblance to the potassium salt.
Ammonium Cyanide NH4CN is formed by the direct union of ammonia
and prussic acid, and can also be obtained by heating a mixture
of sal-ammoniac and potassium cyanide in a retort connected with a
well-cooled receiver. It crystallizes in cubes, smells strongly of ammonia
and prussic acid, and boils at 36°. Its vapour is inflammable,
and bwns with a yellow flame. It is a very unstable compound,
gradually changing into a brown matter, a decomposition which takes
place more quickly in the moist state.
Silver Cyanide, AgCN.—Soluble cyanides produce in a solution of
silver nitrate a white precipitate of silver cyanide, resembling silver
chloride; it is insoluble in dilute nitric acid, but dissolves readily in
ammonia and in potassium cyanide, from which solution the double
salt, AgCN + KCN, can be obtained in octahedrons on evaporation.
This salt is used for electro-silvering.
Zme Cyatride, Zn(CN)2 is obtained as a white precipitate by adding
potassium cyanide to a solution of a zinc salt; it is soluble in an
excess of potassium cyanide.
fficfal Cyanide Ni(CN)a.—Nickel salts are precipitated by potassium
cyanide; the apple-green precipitate produced is readily soluble
in an excess of potassium cyanide solution, forming the double salt,
Ni(CN)o + 2KCN; by adding hydrocliloric acid to a solution of this
salt, nickel cyanide is again precipitated.
Cobalt Cyanide Co(CN)2 is a brownish red nrecipitate, which also
dissolves easily in an excess of potassium cyanide. When this solution
is slightly acidulated and boiled, hydrogen is given off, and potassium
cobalticyanide is formed:—
2Co(CN)2 + 6KCN + 2HCN" = Co2(C3Na)4Ko + H,