A Manual of the Chemistry of the Carbon Compounds

THE CARBON COMPOUNDS, 61
Hydrocyanic acid is also formed by passing the electric discharge
through a mixture of acetylene and nitrogen, thus :—
Hydrocyanic acid is used as a medicine, and is contained in certain
officinal preparations such as laurel water, bitter almond water, &c,
which are obtained by distilling the leaves of the common laurel, or
bitter almouds with water. These plants do not contain the prussic
acid ready formed; but, in common with most of the plants of the
same family, contain amygdalin, a complicated compound which,
under certain circumstances, splits up into sugar, oil of bitter almonds,
and prussic acid.
To estimate the quantity contained in these preparations, to a
measured or weighed quantity an excess of potash solution is added,
and then by means of a burette a solution of silver nitrate containing
6'3 grams in one litre. The silver solution is added, until a permanent
precipitate appears; each cb.ctu. corresponds to two milligrams
of anhydrous prussic acid. In this reaction the double cyanide
AgCN + KON is formed, which is not decomposed by alkalis, and is
soluble in water; but as soon as exactly half tho quantity of prussic
acid present is converted into silver cyanide, one drop more of the
silver solution will produce a permanent precipitate of silver cyanide.
To detect hydrocyanic acid, as in cases of poisoning, the suspected
matter is acidulated with tartaric acid, and the prussic acid distilled
off by means of a water bath. The distillate is made alkaline with
caustic soda, and a mixture of a ferrous and ferric salt (a solution of
ferrous sulphate oxidised by exposure to the air) is added, and then an
excess of hydrochloric acid; prussjan blue remains undissolved if
prussic acid is present; if the quantify contained is very small, the
solution appears first of a green colour, and on standing deposits dark
blue flakes. When dilute prussic acid is mixed with yellow ammonium
sulphide, aud the liquid evaporated over a water bath, ammonium
sulphocyanate is formed; the presence of this body is made
known by means of ferric chloride, which produces a deep blood-red
coloration.
METALLIC CYANIDES.
Potassium Cyanide KCN.—The different modes of formation of this
salt have already beeu mentioned; it is often prodnced in large
quantities in the blast furnaces in which iron ores are smelted with
coke or coal. Pure potassium cyanide is obtained by passing the
vapour of prussic ncid into an alcoholic solution of potash; the salt,
being but slightly soluble in alcohol, crystallizes out in cubes. The
pure compound is also formed by the ignition of potassium ferrocyanide,
which is decomposed at a red heat into potassium cyanide,
iron carbide, and nitrogen:—
Fe(CN)6K4 = 4KCN + FeCs + N2
E 2