Contractions. - Hydrocy, Ac. Hy.-x. Present name. - Hydrocyanic Acid. Hcn. Prussic Acid. For. name: German, Blausaure.

Obtained by distilling yellow Prussiate of Potash with Sulphuric Acid and water. The process given by the British Pharmacopoeia of 1867 is as follows: -

Take of

Yellow Prussiate of Potash.

2 1/4 ounces;

Sulphuric Acid . .................

1 fluid ounce;

Distilled Water ....................

30 fluid ounces, or a sufficiency.

Dissolve the Prussiate of Potash in 10 ounces of the water, then add the Sulphuric Acid, previously diluted with 4 ounces of the water and cooled. Put the solution into a flask or other suitable apparatus of glass or earthenware, to which are attached a condenser and a receiver arranged for distillation; and having put 8 ounces of distilled water into the receiver, and provided sufficient means for keeping the condenser and receiver cool, apply heat to the flask, until by slow distillation the liquid in the receiver is increased to 17 fluid ounces. Add to this 3 ounces of distilled water, or as much as may be sufficient to bring the acid to the required strength, so that 100 grains (or 110 minims) of it, precipitated with a solution of Nitrate of Silver, shall yield 10 grains of dry Cyanide of Silver.

Characters and Tests. - A colourless liquid, with a strong peculiar odour. Sp. gr. .997. It gives no precipitate with Chloride of Barium, but with Nitrate of Silver it gives a white precipitate entirely soluble in boiling concentrated Nitric Acid. Treated with a minute quantity of Sulphate and Per-sulpbate of Iron, afterwards with Potash, and finally acidulated with Hydrochloric Acid, it forms Prussian Blue. 270 grains of it rendered alkaline by the addition of solution of Soda, require 1,000 grain measures of the volumetric solution of Nitrate of Silver to be added before a permanent precipitate begins to form, which corresponds to 2 per cent. of the real acid.

Preparation. - The officinal acid of the British Pharmacopoeia is the strongest that should be dispensed. Equal measures of the officinal acid and rectified spirit will make the first centesimal dilution.

N.B. - Hydrocyanic Acid and its dilutions should be freshly made, as it is apt to deteriorate when kept.

Reference to Horn. Proving. - Hartlaub and Trinks.

Proper forms for dispensing. - Below 2, Tincture only. 2 and upwards, Tincture, Pilules, or Globules.