FORMULA: (Absolute hydrocyanic acid) HCN. MOLECULAR WEIGHT: 26.98.
SYNONYMS: Prussic acid, Acidum hydrocyanatum, Acidum borussicum, Cyanhydric acid.
Diluted hydrocyanic acid is an aqueous solution containing 2 per cent, by weight, of absolute hydrocyanic acid (HCN=26.98) and 98 per cent of water (U. S. P.).
Source.—Scheele discovered this acid in 1782. It is contained in many trees and shrubs of the natural order Rosaceae, principally in the sub-orders Amygdaleae and Pomeae, where it is found in the seeds, barks, leaves and flowers, existing uncombined largely in the fluid portions of the plant. It is this acid which gives to the almond, and to peach seeds and leaves their pleasant and characteristic flavor.
Preparation.—"Potassium ferrocyanide, in coarse powder, twenty grammes (20 Gm.) [309 grs.]; sulphuric acid, eight cubic centimeters (8 Cc.) [130♏]; water, sixty-five cubic centimeters (65 Cc.) [2 fl℥, 95♏]; distilled water, a sufficient quantity. Place the potassium ferrocyanide in a tubulated retort, and add to it forty cubic centimeters (40 Cc.) [1 fl℥, 169♏] of water. Connect the neck of the retort (which is to be directed upward), by means of a bent tube, with a well-cooled condenser, the delivery tube of which terminates in a receiver surrounded with ice-cold water, and containing sixty-five cubic centimeters (65 Cc.) [2 fl℥, 95♏] of distilled water. All the joints of the apparatus, except the neck of the receiver, having been made air-tight by means of well-fitting corks, pour into the retort, through the tubulure, the sulphuric acid, previously diluted with twenty-five cubic centimeters (25 Cc.) [406♏] of water. Gently mix the contents of the retort and then heat it in a sand-bath, so as to keep the liquid in brisk ebullition, until about one-half of its volume has passed over into the receiver. Detach the receiver, and assay a small portion of the contents by the method given below. Then add to the remainder so much distilled water as may be required to bring the product to the strength of 2 per cent, by weight, of absolute hydrocyanic acid. Diluted hydrocyanic acid may also be prepared, extemporaneously, in the following manner: silver cyanide, six grammes (6 Gm.) [93 grs.]; hydrochloric acid, five cubic centimeters (5 Cc.) [81♏]; distilled water, fifty-five cubic centimeters (55 Cc.) [1 fl℥, 426♏]. Mix the hydrochloric acid with the distilled water, add the silver cyanide, and shake the whole together in a glass-stoppered bottle. When the precipitate has subsided, pour off the clear liquid. Diluted hydrocyanic acid should be kept in small, dark amber-colored, cork-stoppered vials, in a cool place"—(U. S. P).
Wittstein gives the following formula for making diluted hydrocyanic acid, which forms quite a permanent acid, one that may be freely exposed to the light for ten or twelve weeks without any apparent change: to 16 ounces of distilled water add 4 ounces of ferrocyanide of potassium; when dissolved, add a mixture, cold, of 12 ounces of alcohol specific gravity 0.840, with 3 ounces of sulphuric acid. Let this mixture stand for 24 hours, with occasional agitation. By means of a strainer separate the crystalline precipitate, introduce the clear liquid into a retort which has an inch in depth of its bottom covered with clean quartz-sand, in order to cheek the thumping during the distillation, and distill off 20 fluid ounces. Reduce the distillate to the proper strength by the appropriate tests.
Description and Tests.—Concentrated or anhydrous hydrocyanic acid is not used in medicine. The medicinal acid is a clear fluid, having a peculiar, penetrating, diffusive odor, and a peculiar, rather disagreeable taste, both the "odor and taste resembling those of bitter-almonds"—(U. S. P.). It is very poisonous (on which account it should be tasted with great caution), is very volatile, imparts a slight red tinge to litmus paper which is not permanent, and is decomposed by the action of light, giving rise to a black substance, paracyanogen. Vials in which it is placed should be small, and of a dark-amber color, and well-stopped with cork. It should always be kept in a cool situation. "It is completely volatilized by heat"—(U. S. P). If the acid strongly reddens litmus it contains some other acid; if it be sulphuric acid, a solution of nitrate of barium, which occasions no precipitate in the pure acid, will yield a white deposit of sulphate of barium, insoluble in nitric acid. If hydrochloric acid be present, nitrate of silver forms a white deposit of chloride of silver insoluble in boiling nitric acid, whereas the white cyanide of silver is soluble in nitric acid at 100° C. (212° F.). It is soluble in water, ether, and alcohol in all proportions. Hydrocyanic acid may be known: 1. By its peculiar odor; 2. by its forming prussian blue when, after having accurately saturated it with caustic potash, a solution of sulphate, and of chloride of iron is added to it, and to the precipitate thus procured some diluted sulphuric or hydrochloric acid be added; 3. the white precipitate of cyanide of silver, caused by the addition of a solution of nitrate of silver, is soluble in boiling concentrated nitric acid; 4. a very delicate reaction is as follows: place a few drops of the suspected liquid upon a watch glass, add a few drops of ammonia and yellow ammonium sulphide, warm on a water-bath to decompose the excess of the latter; when the liquid is colorless, render faintly acid with hydrochloric acid and add some ferric chloride; a blood-red coloration of ferric sulphocyanide will appear if HCN was originally present. The following is Schönbein's test for the millionth part of a drop of hydrocyanic acid in water, or in vapor in the atmosphere: dissolve 3 grammes (46 ⅓ troy grains) of guaiac resin in 100 grammes (1543 ⅓ troy grains) of rectified alcohol. Moisten enough filtering-paper in this solution to absorb the whole of it; the paper should remain white. Also make a solution of sulphate of copper 1 decigramme (1 ½ troy grains) to 50 grammes (771 ⅔ troy grains) of distilled water. When it is desired to use this test, cut off a small strip of this paper, moisten it with the solution of sulphate of copper, and place it in contact with the suspected fluid or vapor, and if hydrocyanic acid be present, the paper becomes blue instantly.
Hydrocyanic acid is incompatible with the mineral acids, the salts of iron, the sulphides, chlorine, the oxides of mercury, of antimony, nitrate of silver, etc. Light decomposes it, hence it should always be kept in bottles that are darkened or covered so as to prevent the rays of light from passing into them. In using this acid medicinally, care should always be taken to procure the diluted preparation. Scheele's medicinal hydrocyanic acid is stronger than the official diluted acid of the United States Pharmacopoeia, as 2 is to 5. A very weak hydrocyanic acid is less liable to decomposition than the stronger preparations, hence it is that such preparations as Scheele's (above) is unfitted for medicinal use, on account of the readiness with which it loses strength. Prof. J. U. Lloyd has shown that a hydrocyanic acid made with a portion of alcohol will retain its properties unaltered and without apparent loss, for at least three years. "If to 1 Cc. of the acid, rendered alkaline by potassium hydrate T.S., a few drops, each, of ferrous sulphate T.S. and ferric chloride T.S. be added, and the mixture then acidulated with hydrochloric acid, a blue precipitate will be formed. To ascertain the percentage strength, mix in a flask (of the capacity of about 100 Cc.) 0.27 Gm. of hydrocyanic acid (obtained by distillation as above directed) with sufficient water and magnesia to make an opaque mixture of about 10 Cc. Add to this 2 or 3 drops of potassium chromate T.S., and then, from a burette, decinormal silver nitrate V.S., until a red tint is produced which does not again disappear by shaking. Each cubic centimeter of silver nitrate V.S. used indicates 1 per cent of absolute hydrocyanic acid. After ascertaining the strength of the distillate, dilute it with distilled water so as to bring it to the strength of 2 per cent of absolute acid. Lastly, test the finished product again, when 1.35 Gm. of it should require, for complete precipitation, 10 Cc. of decinormal silver nitrate V.S.—(U. S. P.).
Action and Toxicology.—Prussic acid is one of the deadliest of known poisons. In small doses it produces in man, salivation, faucial irritation, epigastric warmth, bitter, hot taste, dizziness, light feeling in head, tinnitus, pain in head, numbness, vertigo, dusky countenance, drowsiness, staggering gait, praecordial constriction, and the pulse either increased with palpitation, or decreased in action. From such doses ulceration of the mouth, and salivation have occurred. In poisonous doses its effects are very rapid. When a half ounce of the ordinary solutions (2 to 4 per cent) is taken, the symptoms usually commence with swallowing, or quickly thereafter. The symptoms are rarely delayed beyond one or two minutes (Taylor). Faintness, vertigo, or more commonly insensibility, at once take place. Then follow fixation and glistening of the eyes, with dilatation of the pupils which light fails to affect, flaccid limbs, cold skin with clammy perspiration, dusky, turgid countenance, hot head, convulsive breathing at long intervals (appearing dead during the intervals), pulse imperceptible, involuntary evacuations, respiration slow, deep, and gasping, usually convulsive, though occasionally sobbing or heaving, and if there be profound coma, the breathing is stertorous. Usually in fatal cases, relaxation rather than convulsions, is the rule, though in occasional cases convulsions, or rigidity with set jaws, have been observed. The eyes retain their peculiar luster after death, and the congested state of the gastric membranes, as well as the engorgement of blood in the deeper veins, with empty arteries, and the purplish hue of the skin, are among the post-mortem appearances.
The smallest dose which has produced death is that equivalent to 9/10 grain of the anhydrous acid, or 45 minims of the official solution. Time, 20 minutes; a healthy, adult female. The smallest fatal dose is therefore assumed to be 50 minims of the solution [2 per cent] (Taylor). Taylor compares the death to that of lightning, the patient either dies quickly, or recovers altogether. Seven drachms of the solution killed a physician in four or five minutes; and a single drop of the pure acid in the eye or throat of a strong dog killed the animal in a few seconds. The strong odor of bitter almonds is given off from the poisoned patient before and after death. Death may result from either respiratory or cardiac paralysis. If death does not take place in one hour, the patient will probably recover. The effects of potassium cyanide closely resemble those of prussic acid. In cases of poisoning by hydrocyanic acid, there is seldom time to administer an antidote; but when life is not extinct, we may confidently rely on the antidotes we possess. The theoretical antidote is ferrous sulphate. The best is that proposed in the Lancet for 1844, vol. II., p. 41, by Messrs. T. & H. Smith, of Edinburgh, viz.: In a fluid ounce or two of water, dissolve carbonate of potassium 20 grains, and cause the patient to swallow it; and, immediately following this, administer a solution of sulphate of iron 10 grains, tincture of chloride of iron a fluid drachm, in a fluid ounce of water. This will convert about 2 grains of the strong acid into an insoluble prussian blue. While these are being prepared, the symptoms already produced will be best combated by ammonia inspired from a sponge, or taken, diluted, internally; by chlorine water, used by inhalation, and internally, in teaspoonful doses, diluted with water; cold affusion, dashing the water or pouring it from a height, more especially on the head and along the spinal column; and also artificial respiration and electricity.
Medical Uses and Dosage.—When largely diluted (2 per cent), it has been employed in medicine as a sedative to subdue spasm, and allay nervous irritability. It has been used to relieve revere vomiting and purging, to check colliquative diarrhoea, to cure pertussis and spasmodic coughs, asthma, hysteria, chorea, dyspepsia connected with morbid irritability of the stomach, etc.; also externally in several diseases of the skin. It has likewise been found beneficial in the cough of consumptives, cardiac palpitations, hypertrophy of the heart, and in difficult breathing. It is useful in angina but not so valuable as amyl nitrite or glonoin. Minute doses relieve congestive headache. But, from its volatility, its variability of strength, and its proneness to decomposition, it will very frequently disappoint the expectations of the practitioner, either by inducing fatal symptoms, or being wholly inert. The dose of the diluted acid is from 1 to 3 drops in water, mucilage or syrup.
Specific Indications and Uses.—"Elongated and pointed tongue with reddened tip and edges; uneasy sensations in the stomach" (Scudder), nausea, vomiting, gastric irritation and pain; vertigo; angina pectoris; cough of phthisis (palliative); gastric cough with scanty secretions. In the specific use of this drug, 5 minims of the official acid (2 per cent) are added to 4 fluid ounces of water, and teaspoonful doses of the mixture administered every 2 or 4 hours.