HYDROCYANIC ACID. 
205 
§ 257-1 
§ 257. Medicinal Preparations of Prussic Acid. —The B.P. acid is 
a watery solution of prussic acid ; its specific gravity should be 0-997, 
and it should contain 2 per cent, of the anhydrous acid ; 2 per cent, is 
also the amount specified in the pharmacopoeias of Switzerland and 
Norway, and in that of Borussica (6th ed.) ; the latter ordains, however, 
a spirituous solution, and the Norwegian an addition of 1 per cent, of 
concentrated sulphuric acid. The French prussic acid is ordered to be 
prepared of a strength equalling 10 per cent. 
The adulterations or impurities of prussic acid are hydrochloric, sul¬ 
phuric, 1 and formic acids. Traces of silver may be found in the French 
acid, which is prepared from cyanide of silver. Tartaric acid is also 
occasionally present. Hydrochloric acid is most readily detected by 
neutralising with ammonia, and evaporating to dryness in a water-bath ; 
the ammonium cyanide decomposes and volatilises, leaving as a saline 
residue chloride of ammonium. This may easily be identified by the 
precipitate of chloride of silver which its solution gives on testing with 
silver nitrate, and the deep brown precipitate with Nessler solution. 
Sulphuric acid is, of course, detected by chloride of barium ; formic acid, 
by boiling a small quantity with a little mercuric oxide ; if present, the 
oxide will be reduced, and metallic mercury fall as a grey precipitate. 
Silver, tartaric acid, and any other fixed impurities are detected by 
evaporating the acid to dryness, and examining any residue which may 
be left. It may be well to give the various strengths of the acids that 
have been met with in commerce in a tabular form :— 
British Pharmacopoeia, 
(6th ed.) 
France 
Vauquelin’s Acid 
Scheele’s ,, 
Riner’s „ 
Robiquet’s ,, 
Schraeder’s „ 
Duflos’ „ 
Pfaff’s 
Roller’s 
Switzerland, Norway, 
Per cent. 
and Borussica 
2 
10 
3-3 
. . . 4 to 5 2 
10 
50 
1-5 
9 
10 
25 
In English commerce, the analyst will meet with no acid stronger 
than 5 per cent. 3 
Impure oil of bitter almonds contains hydric cyanide in variable 
quantity, from 5 per cent, up to 14 per cent. There is an official pre¬ 
paration obtained by digesting cherry-laurel leaves in water, and then 
1 A trace of sulphuric or hydrochloric acid should not bo called an adulteration, 
for it greatly assists the preservation, and therefore makes the acid of greater thera¬ 
peutic efficiency. 
2 Strength very uncertain. 
3 A 12 per cent, solution used to be sold by Kahlbaum. 
