When hydrocyanic acid has to be separated from organic substances, such as the contents of the stomach, it is usual to take advantage of its ready volatility. If the acid be not in combination it may be given off so readily as to be detected by a watch glass moistened with nitrate of silver held over the vessel containing the acid; but in order to make sure of its presence or absence the following process should be adopted. The suspected material should be acidulated with pure sulphuric acid so as to insure the prussic acid being in a free state. The substances thus acidulated are to be placed in a retort, distilled over a water bath, and the distillate collected in a cool receiver containing some caustic potass. About one-sixth of the fluid substance should in this way be distilled over, when the liquid in the receiver may be tested by the silver or iron tests, or the vapor as it passes over may be tried with the sulphur test.
1. The peculiar _odor_ of prussic acid is well known, and is a very delicate test, taken in conjunction with others, of its presence.
2. _The Silver Test._—Nitrate of silver yields, with hydrocyanic acid or cyanide of potassium, a white clotted precipitate, (cyanide of silver,) insoluble in cold but soluble in boiling nitric acid. If this precipitate be well dried and heated, cyanogen gas will be given off, which may be known by its burning with a purplish flame. This test is very delicate.
3. _The Iron Test._—Of the liquid collected in the receiver above-mentioned, or the suspected acid liquid, saturated with a few drops of caustic potass, a portion is to be taken, and to this is to be added a small quantity of a solution of sulphate of iron. A dirty brownish or greenish precipitate will fall, consisting of a mixture of the oxide of iron and prussian blue. On adding a few drops of diluted sulphuric or hydrochloric acid, and thus dissolving the oxides, the prussian blue will immediately be made clear if hydrocyanic acid be present.
4. _The Copper Test._—Sulphate of copper added to prussic acid rendered slightly alkaline by potass, gives a greenish-white precipitate, which becomes white by the addition of a few drops of hydrochloric acid to dissolve the blue precipitated oxide of copper.