A solution in water containing 2 per cent. by weight of the anhydrous acid (Scheele's acid contains 4 to 5 per cent., Vauquelin's 3.3 per cent.).

Amygdalin and emulsin, from which the acid is developed, exist together in many plants; in the leaves of the cherry-laurel, the kernels of the peach, almond, cherry, etc. In the mineral kingdom the acid is found in combination as cyanate and cyanide; it occurs also in various animal secretions, and may be obtained by heating nitrogenized organic matter in contact with a base. Scheele discovered the acid in 1782, and is said to have been accidentally poisoned by it.


By distilling, with gentle heat, a mixture of ferrocy-anide of potassium (yellow prussiate of potash) and dilute sulphuric acid. Half the cyanogen passes over into the water of a cooled receiver as hydrocyanic acid, and part remains in combination with potassium and iron as a yellowish-white insoluble double salt (Everitt's salt). Some acid sulphate of potassium is also formed: thus - 2K4FeCy6 + 6H1SO4 = FeK2Fe Cy6 + 6KHSO4 + 6HCy.

Characters And Tests

The pharmacopoeial solution is a colorless volatile liquid of characteristic bitter-almond odor. Its taste has been variously described as "hot and bitter" (Taylor), or "cooling, afterward irritating" (R. W. Smith); sp. gr. 0.997 (nearly that of pure water). If free from other acid it reddens litmus but transiently. It loses strength on exposure to air and light, but that which is prepared by the pharmacopoeial process and kept in dark-colored bottles may be retained for years without perceptible change. Stronger solutions alter more readily, and of the anhydrous acid (which has sp. gr. of 0.697) a part evaporates on paper so quickly as to freeze the rest. Cyanides prevent fermentation, and are fatal to vegetable life (Dumas).

1. The White or Silver Test. - Nitrate of silver gives, with prussic acid solutions, a dense flocculent white precipitate of cyanide of silver, insoluble in cold, soluble in boiling nitric acid - HCy + AgNO3 = AgCy + HNO3. This test may be conveniently applied to the detection of prussic acid vapor by means of two watch glasses, the lower one containing a little of the suspected solution, and the upper one, inverted over it, a few minims of nitrate solution (1 gr. to the oz.): the latter soon becomes opalescent, and when dry, leaves a white stain, showing under the microscope prisms or long plates interlaced. Cyanide of silver, like other insoluble cyanides, may be further tested by placing it in a narrow glass tube drawn out at one end and heating: cyanogen will escape and may be lighted at the pointed end; it burns with a rose-colored flame, having a bluish halo.

2. The Blue or Iron Test. - This is applied by adding to the solution a little liquor potassae and a few drops of a mixed solution of a proto-and per-salt of iron (protosulphate and perchloride are commonly used); a greenish-brown precipitate falls, which, on addition of a little dilute hydrochloric acid, becomes dark or Prussian blue in color. The potassic cyanide, first formed, gives rise to ferrocyanide and afterward to ferric cyanide with the iron salts, thus:

HCy + KHO = KCy + H1O

2KCy + FeSO4 = FeCy2 + K2SO4

4KCy + FeCy2 = K4FeCy6

3(K4FeCy6) + 2(Fe2Cl6) = 12(KCl) + Fe4(FeCy6)3 The acid dissolves any excess of precipitated iron oxides that might obscure the color.

3. The Red or Sulphur Test. - Add to the solution a few drops of ammonia and of yellow sulphide of ammonium; warm gently till colorless, and evaporate slowly; to the residue add a drop of acid solution of perchloride of iron; a blood-red color (sulphocyanide of iron, Fe26CyS) is developed; it is discharged by corrosive sublimate, and thus distinguished from meconic acid. In this test some free sulphur in the ammonium sulphide unites with the alkaline cyanide to form sulphocyanate of ammonia.


The ammonia combines with excess of free sulphur, and forms, among other compounds, sulphydrate of ammonium, which should be removed by boiling and evaporation, and if this be not carried far enough, some of the latter compound remains and gives rise to black sulphide of iron instead of sulphocyanide on addition of the perchloride solution.

These two tests are also applicable to the vapor by means of watch glasses.

4. The Copper Test. - To the liquid, rendered slightly alkaline by liquor potassae, add solution of sulphate of copper; a greenish-white precipitate falls, containing cyanate of potash and of copper with some blue oxide. When this is dissolved by a little hydrochloric acid, the precipitate becomes nearly white.

Absorption And Elimination

Hydrocyanic acid is absorbed to some extent, even through the unbroken skin, especially if a strong solution be applied with friction; from a wound, or from mucous membrane, it is, however, absorbed much more readily. When placed on the tongue or swallowed in the ordinary way, it passes sooner into the circulation than when injected into the stomach, rectum, or vagina (Coullon, Kri-mer). In less than thirty-six seconds after a little of the strong acid is placed on an animal's tongue, it may be detected in the circulating blood (Krimer: Horn's Archiv, 1826): after intravenous injection, also, it quickly produces its effects, but most quickly after inhalation. Guineapigs made to inhale the anhydrous acid for one second, die within fifteen seconds, and strong rabbits exposed to the vapor for three seconds, are destroyed within thirty (Preyer: "Die Blausaure," zw. Theil, 1870, s. 133). The weakly, the young, and the aged among warm-blooded animals are much more easily affected by the acid; while frogs, and all coldblooded creatures, are much less sensitive to its action, and survive toxic doses for several hours. Horses are said to be insusceptible to quantities of one or two ounces (Amory: Boston Journal, 1866).

Although so rapidly poisonous to most animals as it is also to men, there is yet no difficulty in concluding that absorption must precede any general action, and Stille has shown that if a tight ligature be placed round a limb exposed to the acid, constitutional effects do not occur, so long as the local is cut off from the general circulation (vol. ii., p. 222, 3d ed.). Ordinary blood is not essential to its action, for the bloodless "salt frog" exhibits the same symptoms under prussic acid as the normal creature (Lewisson: Reichert's Archiv, 1870.)

Elimination is rapid, and for ordinary medicinal doses is probably complete within an hour; even after a full or poisonous amount, if life can be prolonged for that time, recovery may be hoped for. The acid passes out partly by the saliva, to a slight extent by the kidneys, but mainly by the lungs, as evidenced by the characteristic odor of the breath.

Physiological Action (Internal)

Digestive System

Small medicinal doses - 2 to 5 min. of the officinal acid - seldom exert more than a transient effect of sedative character on the gastric mucous membrane; 10 to 20 min. induce local irritation of the fauces and stomach, with increased flow of saliva and nausea; breathing the vapor, or taking by the mouth 20 to 30 min. or more of the diluted acid, causes such symptoms in a marked degree, though not always immediately (Taylor).

Toxic Action

If, with animals, such doses be used as allow time for a somewhat gradual poisoning, vertigo is noted as an early symptom, with loss of power over the muscles, so that the animal quickly falls; the breathing, at first perhaps hurried and panting, soon becomes slow and difficult, while the heart-beats are rapid and weak. Convulsions mark (according to Preyer) the second stage of cyanic poisoning; they may be tonic or clonic, and affect not only the limbs, but the respiratory muscles and the heart: livid features, protruding shining eyeballs, and congested veins evidence the obstructed circulation, and either death soon comes from asphyxia, or after a period of paralysis and torpor, recovery gradually ensues. Evacuations from the bladder and bowel commonly occur during the unconscious stage, and a peculiar shriek often, though not always, precedes death. Post-mortem, intense congestion of the larger venous trunks and the cerebral membranes is the most marked appearance.

In man, more than 60 min. of the dilute, or 1 gr. of the anhydrous acid, will be usually a fatal dose, though symptoms may not be developed for some minutes; after as much as 1/2 fl. oz., however, they will come on in a few seconds, or even during the act of swallowing. Volition and power may be retained just long enough to walk a few paces, to arrange the bed-clothes, or to cork a phial; but suddenly the subject, if standing, will fall prostrate, often with a scream or in convulsions. Within two minutes he will be insensible, paralyzed, with fixed and glistening eyes, dilated insensible pupils, cold clammy skin, and swollen cyanotic face: the jaw is set, saliva exudes from the mouth, and evacuations occur from the bladder and bowel: the breathing, at first perhaps hurried, soon becomes convulsive and gasping, with long pauses and prolonged expiration: the pulse, after a brief quickening, is soon imperceptible, and death occurs by asphyxia within three to five minutes from the fatal dose.1