Hydrocyanic acid, HCN, dissolved in water, and constituting 2 per cent. by weight of the solution, B.P. A liquid consisting of 2 per cent. of absolute hydrocyanic acid (HCN; 27) and 98 per cent. of water, U.S.P.

Characters. - A colourless liquid with a peculiar odour. Specific gravity, 0.997. It only slightly and transiently reddens litmus-paper.

Reaction. - Treated with a minute quantity of a mixed solution of sulphate and persulphate of iron, afterwards with potash, and finally acidulated with hydrochloric acid, it forms Prussian blue.

Preparation. - By distilling yellow prussiate of potash with H2SO4.

Potassium Ferrocyanide

Everett's Yellow Salt

2K4FeCy6 +


= 6HCy

+ K2Fe2Cy6 +


Half the cyanogen of the ferrocyanide passes over as hydrocyanic acid, while a ferrocyanide of potassium and iron, often called Everett's yellow salt, remains behind along with potassium sulphate.

Impurities. - The most important is want of strength, so that when pre. scribed it has not the desired effect. It loses strength when kept, and therefore the volumetric test is more important than in the case of other acids.

Tests. - A fluid drachm of it evaporated in a platinum dish leaves no fixed residue (no fixed impurities). It gives no precipitate with chloride of barium (no sulphuric acid), but with nitrate of silver it gives a white precipitate entirely soluble in boiling concentrated nitric acid (no hydrochloric acid). 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. Silver nitrate forms a soluble double cyanide of silver and sodium, and till all the hydrocyanic acid is used up no silver oxide is precipitated. AgNo3 + 2NaCy = NaNO3 + NaAgCy2. The silver oxide reacts on the soluble compound, and decomposes it, so that a permanent precipitate of silver cyanide is formed. 2NaAgCy2 + Ag2O + H2O = 2NaHO + 4AgCy.

Standard silver test solution contains 1/10 of an equivalent of AgNO3, and 1,000 grains therefore combine with 1/10 of 2NaCy.

Dose. - 2 to 8 minims. As a lotion, 5-10 min. to 1 fl. oz. of water, rose water, elder flower water, or almond mixture. The addition of 1 fl. dr. of glycerine tends to prevent evaporation.

Preparations. B.P.

Vapor Acidi Hydrocyanici. Tinctura Chloroformi et Morphinae (contains 1 vol. in 16). B.P. Vapor Acidi Hydrocyanici. Vapour of Hydrocyanic Acid. - Mix 10 to 15 minims of diluted hydrocyanic acid with 1 fluid drachm of cold water in a suitable apparatus, and let the vapour that arises be inhaled.

Action. - Hydrocyanic acid differs from all the other acids in having upon the organism an action peculiarly its own. It is one of the most powerful and most rapid poisons known. It destroys protoplasmic movement, kills infusoria, checks oxidation, and arrests fermentation. When applied to the skin it passes through the epidermis and paralyses the ends of the sensory nerves below, so that the part becomes numb, and tactile sensation is diminished or destroyed. It is rapidly absorbed from the mucous membranes, and its action is the same when applied to any of them. A single drop of pure hydrocyanic acid injected into the eye, nose, or mouth of a small animal causes it to fall down dead as if struck by lightning, and the same dose is sufficient to cause the death even of a large animal. In these cases the pupils are usually widely dilated, and the animal generally utters a characteristic cry. When a smaller, but still fatal dose is given, the poisoning may be divided into three stages. In the first stage the brain is chiefly affected. There is giddiness, uncertain gait, a few slow breaths, and then rapid respiration and irregular action of the heart. These are succeeded in the second stage by violent convulsions, tonic and clonic. The head is bent backwards, the limbs are stiffly extended, and sensibility is generally lost, although reflex action may still persist. In the third stage there is coma, complete loss of sensibility, paralysis of the voluntary muscles, almost imperceptible pulse, slow and weak respiration - the expiratory movements predominating, and death.

It is evident that these are the symptoms of rapid asphyxia. They are very like those produced by carbonic acid, but much more rapid, and resemble those of poisoning by sulphuretted hydrogen. The convulsions occur only in warm-blooded animals, and not in frogs. In this point they resemble those of simple asphyxia (p. 237). They differ from those of ordinary asphyxia, however, in the fact that whereas the blood is venous when asphyxial convulsions occur, the blood is arterial in colour when the hydrocyanic acid convulsions occur. They differ also in not being arrested by artificial respiration.

Death, in animals poisoned by hydrocyanic acid, is due to sudden arrest of the heart in the more rapid cases, and to paralysis of the respiration in those which occur more slowly. In consequence of this, the blood in the left side of the heart is found to be arterial in cases of instantaneous death, but venous in those instances where some minutes have been required. It is stated that in the first stage of poisoning the blood is more arterial than usual, though it afterwards becomes more venous. This has been said to depend upon diminution of the oxidising power of the blood by the action of the acid. Hydrocyanic acid is said to form a compound with haemoglobin (cyan-haemoglobin) which does not readily give up its oxygen (p. 70). But this compound is often not to be found in the blood of animals poisoned by the acid, and the arterial appearance is more probably due to dilatation of the peripheral vessels allowing the blood to pass through them rapidly, without undergoing the usual changes, just as it does in the sub-maxillary gland on irritation of the chorda tympani nerve. This is rendered all the more probable by the fact, that at the exact moment in which the blood becomes of an arterial colour in the veins, the blood-pressure suddenly falls in the arteries (Rossbach).

The respiratory changes, however, do seem to be also interfered with, for in the first stage of poisoning the exhalation of carbonic acid is diminished. As the diminution in the power of the blood to give oxygen off is hardly sufficient to explain this, and as the convulsions, apparently asphyxial in character, come on while the blood is still arterial, we may, with some probability, suppose that the respiratory changes are due to the effect of the hydrocyanic acid in lessening internal respiration in the nervous tissues themselves (p. 239).

The stoppage of the heart in mammals is partly due to irritation of the vagus-roots in the medulla, and partly to paralysis of the motor ganglia in the heart.

When placed upon the heart of a frog it arrests its beats, but the heart, at first, still contracts when irritated, though after a short time its muscular irritability is also lost.

That its action in stopping the mammalian heart is partly due to irritation of the vagus-roots is shown by the fact that, in some animals, section of the vagi prevents the stoppage. The effect of hydrocyanic acid is, first to raise, and afterwards greatly to depress the arterial pressure, and at the same time to slow the pulse. The slowing and paralysis of respiratory movements which this acid produces are chiefly due to its action on the respiratory centre in the medulla oblongata. When directly applied to the medulla in the alligator it causes continuous powerful expiration and death, whereas when given in other ways considerable time is required for its action to be produced. It appears to paralyse the brain, peripheral afferent nerves, then spinal cord, motor nerves, and muscles. That the afferent nerves are paralysed before the cord is proved by the fact that when frogs are poisoned with prussic acid, and afterwards with strychnine, slight irritation of the sensory nerve-roots will cause tetanus, after irritation of the periphery has ceased to produce any effect.

This fact was observed by Von Kiedrowski, working under Reichert's direction. The same author observed the effect of the local application of hydrocyanic acid in paralysing muscle and nerve, by removing the soft parts and bones from the lower part of the thigh of a frog, leaving the leg attached to the body only by nerves (Fig. 166). The gastrocnemius and crural muscles were then separated, and the gastrocnemius with its nerve was immersed in aqueous humour diluted with water, and the crural muscles with their nerves in a similar liquid to which hydrocyanic acid had been added. After four hours the crural muscles did not contract on direct irritation, but the gastrocnemius did so readily. This showed that the acid had paralysed the muscles. Irritation of the gastrocnemius, of its nerve f g, or of the sciatic nerve a, caused reflex movements in the body of the frog, but irritation of the crural muscles caused no such reflex movements, showing that the ends of the sensory nerves within them had been paralysed. When the sciatic a was irritated the crural muscles did not contract, but the gastrocnemius did. The poison probably paralyses motor nerves as well as muscles, for it is found that the muscles contract, though feebly, on direct irritation, after they have ceased to respond to the strongest irritation of the motor nerves.

a, the sciatic nerve; b, thigh of a frog; d and e, branches of sciatic going to the crural muscles; f g, branch going to the gastrocnemius.

Fig. 166.   After Kiedrowski. Diagram to show the effect of hydrocyanic acid when applied locally.

Fig. 166. - After Kiedrowski. Diagram to show the effect of hydrocyanic acid when applied locally.

Uses. - Hydrocyanic acid is used externally in order to lessen itching in skin-diseases, and is best applied in combination with glycerine. It is chiefly employed internally to diminish irritability of the stomach, and to relieve vomiting, also pain in the stomach or intestines, and functional palpitation of the heart dependent on dyspepsia. It is also used to relieve cough in cases of bronchitis, phthisis, asthma, and whooping cough. It has sometimes been employed, though with doubtful effect, in chorea, epilepsy, and hysteria. Its vapour is sometimes used to lessen irritability of the respiratory passages and cough.