Hydrocyanic Acid, HCN or HCy, 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.
(1) Chemistry.—Pure anhydrous hydrocyanic acid is a colourless, highly volatile liquid, with a specific gravity of 0.697 at 64° F. It boils at 80°, and solidifies into a crystalline mass at 5° F. It possesses a very penetrating odour, resembling that of peach-blossoms or oil of bitter almonds. It burns with a whitish flame, reddens litmus paper slightly (its acid properties 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 may be obtained by the action of concentrated hydrochloric acid on cyanide of mercury. The diluted hydrocyanic acid of the British and other pharmacopœias is, however, of more practical importance. It is made (British Phar- macop.) by distilling ferrocyanide of potassium with dilute sulphuric acid, and is standardised to a strength of 2 per cent. When kept for any length of time it is extremely apt to decompose.
The ordinary tests for hydrocyanic acid are (1) the peculiar odour; (2) the nitrate of silver test—there being formed a white precipitate of cyanide of silver, which is soluble in boiling nitric acid; (3) the formation of Prussian blue, by adding to the 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. To the suspected liquid add ammonia and yellow sulphhydrate of ammonium; evaporate the liquid in a watch-glass to dryness, occasionally adding ammonia till the excess of sulphhydrate of ammonium is decomposed. Add water, and acidify with hydrochloric acid. If hydrocyanic acid be present the sulphocyanate of ammonium which has been formed gives a blood-red solution on the addition of a ferric salt.
(2) Medicinal Uses.—Diluted hydrocyanic acid is used externally as an ingredient of lotions to diminish itching in skin diseases. In 2 to 8 minims doses it is given internally to diminish irritability of the stomach, to relieve gastro-intestinal pain, vomiting, and functional palpitation of the heart. Given by the mouth or by inhalation it is also useful in allaying cough in phthisis, hooping-cough, bronchitis, &c. All these applications depend upon its action in deadening sensory nerves.
(3) As a Poison.—Hydrocyanic acid is one of our most energetic poisons, and is frequently employed both for murder and suicide. When a small poisonous dose (about half a drachm of the 2 per cent. acid) has been taken the first symptoms are weight and pain in the head, with confusion of thought, giddiness, nausea (and sometimes vomiting), a quick pulse, and loss of muscular power. If death result this is preceded by convulsions and involuntary evacuations. When a large dose has been taken (as from half an ounce to an ounce of the 2 per cent. acid) the symptoms may commence in a few seconds, and it is seldom that their appearance is delayed beyond one or two minutes. 'When,' says Dr A. S. Taylor, 'the patient has been seen at this period he has been perfectly insensible, the eyes fixed and glistening, the pupils dilated and unaffected by light, the limbs flaccid, the skin cold and covered with a clammy perspiration; there is convulsive respiration at long intervals, and the patient appears dead in the intermediate time; the pulse is imperceptible, and the respiration is slow, deep, gasping, and sometimes heaving or sobbing.' The patient survives for a longer or shorter period, according to the dose. According to Dr Lonsdale, death has occurred as early as the second and as late as the forty-fifth minute; the poison acts as a paralysant to the whole nervous system. Death is due to paralysis of the heart in the more rapid cases, and to paralysis of the respiration in those which occur more slowly.
Where the fatal action is so rapid antidotes are of comparatively little value. Chlorine, ammonia, cold affusion, and artificial respiration are the most important agents in the treatment. The first two should be used with great caution, and only by the medical practitioner. Cold affusion on the head, neck, and down the spine is a valuable remedy. Artificial respiration (see RESPIRATION, ARTIFICIAL) should never be omitted.