HYDROCYANIC ACID or Cy,H), known also as prussic acid, from its having been first obtained by Scheele, in 1782, from the substance known as Prussian or Berlin blue, is of almost equal interest to the chemist, the physician, and the toxicologist. We shall notice (1) its chemistry, (2) its medicinal value, and (3) its action as a poison, and its antidotes.
1. Its Chemistry.—Pure anhydrous hydrocyanic acid is a limpid volatile fluid, with a specific gravity of 0.697 at 64° F. It boils at 80°, and solidifies into a crystalline mass at 5° F. Its volatility is so great that if a drop be allowed to fall on a piece of glass, part of the acid becomes frozen by the cold produced by its own evaporation. It pos sesses a very penetrating odor, resembling that of peach-blossoms or oil of bitter almonds. It burns with a whitish flame. reddens litmus paper slightly (its acid proper ties being feeble), and is very soluble in water and alcohol. Pure hydrocyanic acid may be kept unchanged if excluded from light, which occasions its decomposition, and the formation of a brown substance known as paracyanogen.
Hydrocyanic acid is readily obtained by distillation from the kernels of bitter almonds, and many kinds of stone-fruit, from the leaves and flowers of various plants, and from the juice of the tapioca plant (jatropha manihot). Anhydrous hydrocyanic acid is obtained by the reaction of concentrated hydrochloric acid on cyanide of mer cury.
The preparation of the dilute acid is, however, of much greater practical impor tance. The London, Edinburgh, Dublin, and United States pharmacopndas agree in recommending that it should be obtained by the distillation of a mixture of dilute sul phuric acid and ferrocyanide of potassium (known also as prussiate of potash). The distillate should contain nothing but hydrocyanic acid and water, so that, by the addi tion of more water, we can obtain an acid of any strength we please. Sometimes, how ever, a second, or even a third distillation is necessary. The dilute acid of the Ph. Land. contains 2 per cent; that of the Ph. Dub. rather more; that of the Ph. Edin. con tains 3.3 per cent; while what is known as Scheele's acid is very variable, but averages 4 per cent of the anhydrous acid.
The ordinary tests for hydrocyanic acid are 1, the peculiar odor; 2, the nitrate of silver test—there formed a white precipitate of cyanide of silver, which is soluble in boiling nitric acid; 3, the formation of Prussian blue, by adding to fluid under examination a solution of some proto and per-salt of iron, by then saturating with caustic potash, and finally adding an excess of hydrochloric acid; when, if hydrocyanic acid is present, we have a characteristic blue precipitate; 4, the sulphur test, which is the best and most accurate that has yet been discovered. Let the suspected liquid be
acidulated with a few drops of hydrochloric acid: place it in a watch-glass, and let a second watch-glass, moistened with a drop of a solution of hydrosulphate of ammonia, be inverted over it; after a few minutes, let the upper glass be removed, and the moistened spot be gently dried. The whitish film which is may consist merely of sulphur; when hydrocyanic acid is present, it consists of sulphocyanate of ammonia. Let this residue he treated with a drop of a weak solution of perchloride of iron, when, if hydrocyanic acid was present, a blood-red tint is deVeloped, which disappears on the addition of one or two drops of a solution of corrosive sublimate. This is known as Liehig's test.
2. Its Medicinal are indebted to the Italians for the introduction of hydro cyanic acid in the materia medica; and it was first employed at the beginning of the present century. There are no cases in which it is so serviceable as, in affections w of the stomach in which pain IS a leading symptom, as in gastr6dYnla; water-brash, and in cases of intense vomiting. Hence it is often useful in English cholera, when opium has completely failed. In pulmonary diseases it does not produce the good effects that were formerly ascribed to it; but it is sometimes useful in allaying spasmodic cough. It has been employed with advantage in chronic skin-diseases, to allay pain and irrita tion. A mixture of two drams of the dilute acid (of 2 per cent strength) with half a pint of rose-water, and half an ounce of rectified spirit, forms a good lotion. When given internally, the average dose is from 3 to 5 minims of the 2 per cent dilute acid,' three or four times a day; it must be administered in some milk vehicle, such as simple water, or orange-flower water.