[252] Neues Repert. de Pharm., 18, 356.
[253] 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 Toxicologie, p. 92.
(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.[254]
[254] M. Carey Lea, Amer. Journ. of Science [3], ix. pp. 121-123; J. C. Society, 1876, vol. i. p. 112.
(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.
(7.) Schönbein’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 becomes of a bright pure red and froths strongly; if, on the other hand, a trace of prussic acid be present, the liquid becomes brown and does not froth, or only slightly does so.