Prop. The aqueous is very similar in properties to the anhydrous acid, differing in taste, odour, poisonous and combustible properties, according to its degree of concentration. Like the anhydrous, the aqueous acid decomposes, but not so readily; when perfectly pure, becoming brown, and at last black. As before stated, a little free mineral acid assists to preserve it. It should be always kept in a dark place.
Detection and estimation of hydrocyanic acid and soluble cyanides.—The presence of hydrocyanic acid, indicated by the characteristic smell, which is given off by the contents of the stomach, or of any fluid containing it (provided this is not disguised by any substance of stronger odour) may be confirmed by the following tests:
1. To the filtered suspected fluid add a slight excess of caustic potash, and then a solution containing ferrous and ferric sulphates. If hydrocyanic acid, or a soluble cyanide be present, upon the addition of an excess of hydrochloric acid the liquid turns to a blue colour (more or less intense according to the quantity of acid present), owing to the formation of Prussian blue.
2. Add the suspected fluid solution of nitrate of silver; if hydrocyanic acid be present, a white cyanide of silver is formed, which is nearly insoluble in cold nitric acid, but is soluble in ammonia and cyanide of potash, and which, when heated to redness, gives off the inflammable violet-flamed cyanogen.
3. Acidulate a small quantity of the suspected liquid with a few drops of hydrochloric acid, and place it in a watch-glass; then invert a second watch-glass, moistened with a drop of solution of ammonic sulphide over this. After a few minutes remove the upper watch-glass, and evaporate the liquid to dryness over a water bath; let the dry residue be treated with a drop of a weak solution of ferric chloride. If hydrocyanic acid be present, a blood-red colour is produced, owing to the formation of red ferric hydrocyanide, which may be discharged by chloride of mercury; a reaction which distinguishes it from a similar colour given by meconic acid.
Where large quantities of material have to be examined, it is desirable that the acid should be distilled off by the heat of a water-bath, acidulating the liquid with tartaric acid if it be alkaline. The distillate is then to be tested by any of the above methods.
Antidotes.—Give a scruple of carbonate of potash dissolved in about an ounce of distilled water, and directly afterwards, ten grains of sulphate of iron, also dissolved in the same quantity of distilled water, to which should be added one drachm of tincture of perchloride of iron. Whilst this is being prepared, and subsequently, apply cold affusion to the head and neck, artificial respiration, and, if practicable, give strong coffee and brandy. A more ready remedy is ammonia, given both internally and applied to the nostrils.
Hydrocyanic Acid, Diluted. Syn. Acidum Hydrocyanicum Dilutum. (B. P.) The ‘British Pharmacopœia’ gives the following simple directions for the preparation of this acid:—
Dissolve two and a quarter ounces (avoir.) of ferro-cyanide of potassium in ten fluid ounces of distilled water, then add one fluid ounce of sulphuric acid, previously diluted with four fluid ounces of distilled water, and cooled. Put them into a retort and adapt this to a receiver, containing eight fluid ounces of water, which must be kept carefully cold. Distil with a gentle heat until the fluid in the receiver measures seventeen fluid ounces. Add to this three fluid ounces of the water, or as much as may be sufficient to bring the acid to the required strength of two per cent. by weight.
Prop. Specific gravity, ·997. 100 grains, or 110 minims, precipitated with a solution of nitrate of silver, give a precipitate of cyanide of silver, which when dried, weighs 10 grains. 270 grains rendered alkaline by liquor sodæ, require a 1000 grain measures of volumetric solution of nitrate of silver, before a permanent precipitate begins to form.