HYDROCYANIC ACID

Hydrocyanic acid is a compound of cyanogen and hydrogen. It was first obtained by Scheele in 1782, but it was not until 1815 that Gay-Lussac pointed out its real nature. Anhydrous hydrocyanic acid may be obtained by passing over cyanide of mercury, gently heated, a stream of dry sulphuretted hydrogen. It is now made by mixing ferrocyanide of potassium with dilute sulphuric acid, and applying heat, when the acid is distilled over and collected in a cooled receiver.

Dilute hydrocyanic acid, the only important form of the acid from a toxicological point of view, is a colourless, feebly acid liquid, with a peculiar odour, like that of bitter almonds or peach kernels (specific gravity, 0.997). The Pharmacopœial acid contains 2 per cent. of anhydrous acid; that of Scheele 5 per cent. According to Taylor, however, the percentage of the acid varies from 1.3 to 6.5. Taking into consideration the smallness of the dose, and the shortness of the time before death occurs, it is the most deadly of all known poisons. Prussic acid is not regarded as a cumulative poison—that is, it does not gradually accumulate in the body and then break out with dangerous or fatal violence.

Symptoms.—These will be more or less modified by the quantity of the dose, and in some cases closely resemble an attack of epilepsy. In most cases, the symptoms of poisoning are seldom delayed beyond one or two minutes; and if the dose be large, the symptoms of poisoning may come on while the person is drinking. Giddiness, followed by almost complete insensibility, mark the accession of the symptoms. The eyes are fixed, staring, and glassy; the pupils are dilated, and insensible to light. The muscles of the extremities are relaxed, and the limbs flaccid. A white or bloody froth surrounds the mouth, and the jaws are fixed. The surface of the body is cold and clammy to the touch; the respiration is sometimes long-drawn and spasmodic; and the pulse so reduced as to be almost imperceptible. The breathing is sometimes stertorous in character. This is an important fact; for, in ignorance of the occasional presence of this symptom, it was argued that Walter Palmer, whose breathing was stertorous, died of apoplexy, and not from prussic acid as was alleged. When the dose is small (between twenty and thirty drops of the dilute acid), the patient complains of nausea, giddiness, and a feeling of constriction round the head. The mind is confused, the pulse hurried, and the breathing irregular. Salivation may also be present. Tetanic spasms and involuntary evacuations precede the fatal termination. When the dose is from ten to twenty drops, the patient complains of nausea, giddiness, and a feeling of impending suffocation. These symptoms under treatment may soon pass off, or leave the patient more or less confused and listless. In most cases, where the dose is very large, death takes place suddenly, without convulsions; but the period of death does not appear to be as short in man as in the lower animals.

External Application.—Applied to the unbroken skin, prussic acid does not appear to have caused any alarming symptoms; but it should be used with the utmost caution where the skin is at all abraded or ulcerated.

Post-mortem Appearances.—In making an inspection, care should be taken; for, if the dose be large, the vapour from the corpse on opening it has been known to produce giddiness and fainting. Externally, the skin is pale, livid, or of a violet colour. The hands are clenched, and the nails blue. The jaws are firmly set, and there is usually some froth around the mouth. The internal organs are greatly congested, and the venous system gorged with fluid dark-coloured blood. The stomach and intestines are sometimes inflamed, but in many cases they present no material alteration in colour.

The appearances, when only a small dose has been taken, are not unlike those of asphyxia. The detection of the odour of hydrocyanic acid in the body is of importance; but this may be absent from the following causes:

1. Smallness of the quantity of the acid present.

2. Volatilisation from exposure of the corpse to the air.

3. The smallness of the dose, and its absence the result of absorption and elimination, if death has not rapidly taken place.

4. The amount of dilution of the poison.

5. Concealed by other odorous substances.

In some cases, the smell may be detected in the stomach seven or eight days after death. The viscera should, in all cases of suspected poisoning, be placed in a glass-stoppered jar, and the stopper covered by bladder and tinfoil. Hydrocyanic acid is so volatile that, unless the greatest care be taken, all traces of it may vanish; and thus the guilty person may be allowed to escape.

Fatal Period.—From a few seconds to as many minutes. Under active treatment, if a patient survive forty minutes, he will generally recover.

Fatal Dose.—Thirty minims of the dilute acid of the Pharmacopœia. This contains six-tenths of a grain of the anhydrous acid. Recovery has, however, taken place even after comparatively large doses. The strength and age of the individual, and also the emptiness or fulness of the stomach at the time the poison is swallowed, will materially affect the issue.

Experiments on Animals

Numerous experiments on animals have been made to ascertain the rapidity with which prussic acid kills. The late Sir R. Christison found that three drops projected into the eye acted on a cat in twenty seconds, and killed it in twenty more. The same quantity dropped on a fresh wound in the loins acted in forty-five, and proved fatal in one hundred and five seconds. In the cases where death did not occur so rapidly, there were regular fits of violent tetanus; but in the very rapid cases, the animals perished, just as the fit was ushered in, with retraction of the head. In rabbits opisthotonos, in cats emprosthotonos, were the chief tetanic symptoms.

As a proof that the acid acts equally on the brain and spinal cord, may be noticed the presence of coma and tetanus in some cases of poisoning by this substance.

In the experiments on animals certain effects were noticed, which are as follows:

Expulsion of the Fæces and Urine.—In some cases only the fæces, in others the urine alone, was involuntarily expelled; and in some other cases neither the one nor the other was present.

The Shriek or Cry.—This cry, though a common, is by no means a constant symptom.

Convulsions.—These are sometimes present.

Acts of Volition.—Only slight acts are possible; in the case of one of the dogs experimented on by Mr. Nunneley, the animal “went down, came up, and then went down again the whole flight of a steep, winding staircase.”

The Post-mortem Appearances were not well marked in the animals subjected to experiment. In chronic cases, Mr. Nunneley states that both sides of the heart were distended with black blood. The pure acid is stated to completely destroy the irritability of the heart and voluntary muscles, galvanism producing no effect whatever. “In eight experiments on cats and rabbits with the pure acid, the heart contracted spontaneously, as well as under stimuli, for some time after death, except in the instance of the rabbit killed with twenty-five minims, and one of the cats killed by three drops applied to the tongue. In the last two the pulsation of the heart ceased with the short fit of tetanus which preceded death; and in the rabbit, whose chest was laid open instantly after death, the heart was gorged, and its irritability utterly extinct.”

Detection of Hydrocyanic Acid
in Cases of Poisoning

The “Vapour Tests” are those most readily applied to organic mixtures; but in some cases it may be necessary to make a distillation of the suspected substance, in order to isolate the poison.

The first point to be noticed is, whether any odour of the acid can be perceived in the substance under examination. In any case, the contents of the stomach or finely-divided tissues should be mixed with water, and examined as to the reaction with test paper. If the mixture be found to be alkaline, it must be neutralised by the addition of tartaric acid; if, on the contrary, it be acid, carbonate of soda must be carefully added to neutralisation. A state of neutrality is always necessary previous to distillation, for the following reasons:

An alkaline state of the liquid would, on the one hand, prevent, or, at all events, retard, the evolution of the hydrocyanic acid; whilst, on the other, the existence of any free acid would decompose any cyanide which might be present, and thus give rise to an evolution of hydrocyanic acid not existing as such in the mixture.

The organic mixture is then placed in a flask, and the contents distilled at as low a temperature as possible by the aid of a water bath.

Should hydrocyanic acid be present, the distillate will yield all the characteristic reactions of the dilute acid.

1. Nitrate of silver will give a curdy-white precipitate, insoluble in cold but soluble in boiling nitric acid. A portion of the precipitate, on the addition of some liquor potassæ, sulphate of iron, ferric chloride and hydrochloric acid, forms Prussian blue. In this test, which may be taken as quite conclusive, the hydrochloric acid decomposes the cyanide of silver; and on the addition of the sulphate of iron, Prussian blue is formed.

2. If a portion of the dry precipitate formed by the nitrate of silver be heated in a test tube, cyanogen gas will be evolved, known by its characteristic odour of peach blossoms, and by its burning at the mouth of the tube with a rose-coloured flame.

3. To the solution containing hydrocyanic acid add a few drops of potassium nitrite, two or three drops of ferric chloride solution and dilute sulphuric acid until a yellow tint is obtained; heat to boiling, cool, precipitate excess of iron with ammonia, filter, and add one or two drops of a very dilute solution of colourless ammonium sulphide. A very minute quantity of hydrocyanic acid gives a violet-red colour, changing to blue, green, and finally yellow.

4. If a solution of starch be tinged blue with iodine, the colour will be discharged by a minute quantity of hydrocyanic acid.

Vapour Tests.—There are three tests for the presence of hydrocyanic acid when present in organic mixtures, which have the advantage of being applicable without the addition of anything extraneous to the mixture to be tested. They are all dependent on the volatile nature of hydrocyanic acid, and may be applied as follows, the suspected mixture being divided into three portions:

1. Iron or Prussian Blue Test.—The liquid mixture to be tested is placed in a small beaker glass, and covered with a glass plate the centre of which is smeared with a mixture of potash and proto-sulphate of iron. The whole is now left undisturbed for some time. The glass is eventually removed, and the mixture of potash and iron treated with hydrochloric acid, which, should hydrocyanic acid be present, will cause the development of Prussian blue.

2. Sulphur Test, or Liebig‘s Test.—A second portion of the original mixture is placed in a beaker, and a watch-glass containing a few drops of bisulphide of ammonium is suspended over the liquid, the mouth of the beaker being closed. A short time is allowed to elapse; the watch-glass is then removed, and its contents evaporated to dryness at a low temperature. A blood-red colour is developed on the addition of a little perchloride of iron to the dry residue. This effect is due to the absorption of the hydrocyanic acid vapour by the bisulphide of ammonium—sulphocyanide of ammonium being formed, which, on the addition of perchloride of iron, gives the blood-red colour of the sulphocyanide of iron, which is bleached by corrosive sublimate.

3. Silver Test.—This is the most successful of the vapour tests, a single apple pip yielding all the reactions. If a watch-glass containing a few drops of nitrate of silver solution be suspended in a beaker (as in 2), the silver solution will become white and opaque, from the formation of cyanide of silver; examined under the microscope it is seen to consist of small prismatic crystals. The cyanide as formed, treated with hydrochloric acid, liquor potassæ, and sulphate of iron, will give Prussian blue.

The Quantitative Analysis.—Use the precipitate of cyanide of silver, 100 grains being equal to 20.33 of pure anhydrous acid.

Treatment.—The treatment of poisoning by prussic acid is now to be considered. As part of the general treatment, the stomach pump should be at once employed, and the stomach emptied and then washed out with water.

Ammonia.—The use of this substance was first advocated by Mr. John Murray of London, and is no doubt a valuable remedy if given early. Care should be taken that the mucous membrane of the air passages and alimentary canal be not inflamed by using too strong a solution.

Chlorine.—Recommended by Riauz in 1822. Water impregnated with the vapour of chlorine may be given internally, and the gas may be breathed under proper precautions.

Cold Affusion.—First proposed by Dr. Herbst of Göttingen. Its success is most to be looked for when it is employed before the convulsive stage of the poisoning is over. The cold water should be poured on the head and down the spine.

Fig. 39.—Photo-micrograph of crystals of cyanide
of silver obtained by the vapour test, × 50.
(R. J. M. Buchanan.)

Bleeding from the Jugular Vein.—In one case treated by Magendie, bleeding from the jugular vein was attended with success.

Chemical Antidotes.—The administration of a solution of carbonate of potash, followed by a solution of the mixed sulphates of iron, has been suggested. The formation of Prussian blue is the result. The only objection to this treatment is, that prussic acid is so rapidly absorbed that death may result from the already absorbed acid before the antidote can be given.

Atropine.—This should be given hypodermically.

Peroxide of hydrogen should be given freely by means of the stomach tube.

Cobalt nitrate 0.5 to 1 per cent. solution has been advocated hypodermically.

Sodium thiosulphate, in 10 per cent. solution, repeated hypodermically.

Cyanide of Potassium

This substance is used largely by photographers and electro-platers. It acts as a poison in a similar manner to hydrocyanic acid, and the symptoms are the same. As a commercial preparation it frequently contains undecomposed potassium carbonate, and may exert a corrosive action on the mucous membranes of the mouth and stomach, leading to the production of blood-stained mucus in the stomach.

Post-mortem Appearances.—These are the same as those described under hydrocyanic acid, with the addition of the corrosive effects.

Fatal Dose.—Five grains have proved fatal in a quarter of an hour; recovery has taken place after forty grains.

Chemical Analysis.—Same as for hydrocyanic acid.

Treatment.—As for hydrocyanic acid.

PREPARATIONS CONTAINING
HYDROCYANIC ACID

The following plants contain prussic acid, and are therefore more or less poisonous in proportion to the quantity of the acid which they severally contain:

Bitter Almonds

The essential oil of bitter almonds is very poisonous. “The oil does not, like common essential oils, exist ready formed in the almond, but it is only produced when the almond pulp comes in contact with water. It cannot be separated by any process whatever from the almond without the co-operation of water—neither, for example, by pressing out the fixed oil, nor by the action of ether, nor by the action of absolute alcohol. After the almond is exhausted by ether, the remaining pulp gives the essential oil as soon as it is moistened; but if it is also exhausted by alcohol, the essential oil is entirely lost. The reason is, that alcohol dissolves out a peculiar crystalline principle named Amygdalin, which, with the co-operation of water, forms the essential oil by reacting on a variety of the albuminous principle in the almond, called Emulsin, or Synaptase.

The essential oil of bitter almonds may contain from 6.0 to 14.33 per cent. of hydrocyanic acid. Deaths from the incautious use of this oil for flavouring articles of confectionery are not infrequent. As the flavour is not in the least injured, it has been suggested to subject the oil to repeated distillation with caustic potassæ, by which means the oil is purified from prussic acid.

Symptoms in Man.—Nausea, vomiting, and diarrhœa, due to gastric irritation, have occurred when the dose has been small, as is the case when confectionery owes its flavour to the use of the essential oil. Idiosyncrasy may have something to do with these effects, for cases are on record where a single almond has produced a state resembling intoxication, followed by an eruption not unlike urticaria or nettle-rash. Taken in large doses, the symptoms produced are identical with those described under poisoning by prussic acid. The breath is usually strongly impregnated with the odour of bitter almonds.

Symptoms in Animals.—Vomiting, trembling, weakness, paralysis, tetanic convulsions, and coma.

Post-mortem Appearances.—These are identical with those seen in poisoning by the pure acid.

Fatal Dose.—The essential oil is from four to eight times as strong as the acid of the Pharmacopœia. From twenty to thirty drops have proved fatal. Death may take place in half an hour or less.

Treatment.—The same as that recommended under prussic acid.

Cherry-Laurel

The cherry-laurel, Prunus Laurocerasus—the leaves of which have been used for flavouring custards, &c.—contains prussic acid, and is therefore poisonous.

In the British Pharmacopœia there is an Aqua Laurocerasi—laurel water—prepared from the leaves. It contains 0.1 per cent. of hydrocyanic acid. It should be used with extreme caution, as the amount of hydrocyanic acid contained in the leaves is uncertain. Death has frequently resulted from its use. The most important case, however, is that of Sir T. Broughton. His mother, who gave him his usual draught on the morning of his death, observed that it had a strong smell of bitter almonds. Two minutes after he took it she observed a rattling or gurgling in his stomach; in ten minutes more he seemed inclined to doze; and five minutes afterwards she found him quite insensible, with the eyes fixed upwards, the teeth locked, froth running out of his mouth, and a great heaving at his stomach, and gurgling in his throat. He died within half an hour after swallowing the draught. No light was thrown on the case by the carelessly conducted post-mortem; but the suddenness of his death, the improbability of apoplexy occurring at so early an age, and the odour of bitter almonds observed by his mother, pointed out clearly enough the true cause of death.