CHAPTER III.
CHEMICAL NOTES.
Note I.—Hydrocyanic or Prussic Acid.
Nature of—Strength of different preparations of, English and foreign—Where found—Tests, preliminary: (1) Odour—(2) Silver—(3) Prussian blue—(4) Sulphur—(5) Guaiacum—(6) Uranium—(7) Picric acid—(8) Cupric sulphate—(9) Cobalt chloride—(10) Mercuric oxide—(11) Peroxide of hydrogen—(12) Mercurous nitrate. Test apparatus—Salts of hydrocyanic acid: (1) Potassium cyanide—(2) Mercuric cyanide—(3) Cyanides of the heavy metals—(4) Double cyanides—(5) Sulphocyanides—Oil of bitter almonds—Antidotes—Fatal dose—Symptoms—Post-mortem appearances—Drops and minims—Period after death at which hydrocyanic acid can be discovered—Formic acid to be tested for—Processes.
Synonyms.—Cyanhydric or prussic acid, Hydric cyanide, Hydrogen cyanide, Acidum borussicum, Blausaüre, Berlinerblausaüre.
Formula HCN, i.e., a compound of single atoms, of hydrogen, carbon, and nitrogen, in the proportions by weight of 1 + 12 + 14 = 27. In its pure state (anhydrous, or free from water), it is a feebly acid, colourless, mobile liquid, inflammable and very volatile. Boiling point 24·5° C. Much lighter than water: sp. gr. ·7058. It has a characteristic overpowering and oppressive odour, resembling peach-blossom or laurel-water. But the anhydrous acid, from its volatility and dangerous character is rarely seen or made. In commerce it is always found as a dilute aqueous solution, the varying strengths in real HCN being:—
In this country, only Scheele’s, and the British Pharmacopœia (2 per cent.) acid, are usually met with.
These numbers, however, must be regarded merely as rough approximations for two reasons; first, on account of the extreme volatility of the acid—if loosely stoppered, or frequently opened, it rapidly loses strength—second, both the anhydrous acid and its aqueous solution are decomposed by light, with formation of a brown matter. This change is supposed to be retarded by a trace of mineral acid, hence a little hydrochloric or sulphuric acid is frequently added to the commercial solution with this object. But the acid may even be stronger than supposed, as the methods of preparation are somewhat various, and the one adopted may have been carelessly carried out. Scheele’s acid is said to be the most popular among medical men; samples of it obtained from different large firms and examined by the author showed very irregular strengths, the lowest being 2, and the highest 8 per cent. The latter was purchased at the shop of a chemist who said he had made it himself, and could guarantee it was of full strength. He had evidently made allowance for deterioration. Woodman and Tidy found 16 samples sold in one neighbourhood as B.P. acid to contain 0·6 to 3·2 per cent. of HCN; others have found 0·25 per cent. not infrequent. It follows that if, in a poison case, a bottle has been found of a hydrocyanic preparation of a definite name, or even with a certain strength or dose marked on it, it will not be safe to trust to such figures without actually determining the amount. In Ball’s trial (Lewes, 1860), the judge asked whether this variation in strength would not make the difference between a medicinal and a poisonous dose? It would not, as the maximum medicinal dose, 4 grains, of even the abnormally strong (8 per cent.) Scheele’s acid mentioned above, would only contain 0·32 grain of anhydrous HCN, and it requires at least half a grain to cause death, while about 1 grain is the usual fatal quantity. And a medical man would not even give the maximum medicinal dose as a beginning, and without precaution.