HYDROCYANIC ACID. 
219 
§ 27 1.] 
to dryness on the water-bath, a little water added, and then a small 
drop of solution of chloride of iron. If hydrocyanic acid is present, 
the sulphocyanide of iron will be formed of a striking blood-red colour. 
A variation of this test is as follows :—A solution suspected to contain 
free hydrocyanic acid is boiled with ammonium sulphide for a few 
minutes, and then concentrated to 1 c.c., acidified with hydrochloric 
acid, and extracted with ether ; the ethereal extracts are allowed to 
evaporate spontaneously, and the residue is treated with very dilute ferric 
chloride solution. This latter is added drop by drop until the addition 
of a drop does not increase the colour. Any red colour is probably due 
to ammonium thiocyanate. Ferric meconate, similar in colour, is 
insoluble in ether; hence the red liquid should be treated with ether; 
if hydrocyanic acid is present, the ethereal solution is coloured violet. 
The test will detect *05 mgrm. HCN. 1 
4. The reaction usually called Schonbein’s, or Pagenstecher and 
Schonbein’s 2 (but long known, 3 and used before the publication of their 
paper), consists of guaiacum paper, moistened with a very dilute solution 
of sulphate of copper (1 : 2000). This becomes blue if exposed to the 
vapour of hydrocyanic acid. Unfortunately, the same reaction is pro¬ 
duced by ammonia, ozone, nitric acid, hypochlorous acid, iodine, bromine, 
chromate of potash, and other oxidising agents, so that its usefulness is 
greatly restricted. 
5. A very delicate test for prussic acid is as follows :—About one- 
half centigrm. of ammonia, ferrous sulphate (or other pure ferrous 
salt), and the same quantity of uranic nitrate, are dissolved in 50 c.c. of 
water, and 1 c.c. of this test liquid is placed in a porcelain dish. On 
now adding a drop of a liquid containing the smallest quantity of 
prussic acid, a grey-purple colour, or a distinct purple precipitate, is 
produced. 4 
6. A hot solution of potassic cyanide, mixed with picric acid, 
assumes a blood-red colour, due to the formation of picro-cyanic acid. 
Free HCN does not give this reaction, and therefore must first be 
neutralised by an alkali. Or the vapours may act on a drop of sodium 
picrate. 
7. Schonbein’s Test.— To a few drops of defibrinated ox-blood are 
added a few drops of the carefully neutralised distillate supposed to 
contain prussic acid, and then a little neutral peroxide of hydrogen is 
added. If the distillate contains no prussic acid, then the mixture 
1 R. Lavialle and L. Varenne, J. Pharm. Chim., 1915. 
2 Neues Repert. de Pharm., xviii. 356. 
3 This reaction (with tincture of guaiacum and copper) has been long known. 
“ I remember a pharmaceutist, who attended my father’s laboratory, showing me 
this test in 1828 or 1829.”-—Mohr’s Toxicoloyie, p. 92. 
4 M. Carey Lea, Amer. Journ. of Science [3], ix. pp. 121-123 ; J. C. Society, vol. i, 
p. 112, 1876. 
