§ 398. Identification of the Alkaloid.—A residue containing strychnine, or strychnine mixed with brucine, is identified—
(1.) By its alkaline reaction and its bitter taste. No substance can possibly be strychnine unless it tastes remarkably bitter.
(2.) By the extremely insoluble chromate of strychnine, already described.[447] A fluid containing 1 : 1000 of strychnine gives with chromate of potash (if allowed to stand over-night) a marked precipitate, dissimilar to all others, except those of lead and baryta chromates, neither of which can possibly occur if any of the processes described are followed.
[447] 1 grm. of strychnine gave 1·280 grms. of the chromate, = 78·1 per cent. of strychnine; 3 gave 3·811 of the chromate, = 78·77 per cent. of strychnine.—Mohr.
(3.) If the chromate just described is treated on a porcelain plate with a drop of pure strong sulphuric acid, a deep rich blue colour, passing through purple into red, rapidly makes its appearance. This colour possesses an absorption spectrum (figured at [p. 55]). Dr. Guy, neglecting intermediate colours, aptly compares the succession—(1) to the rich blue of the Orleans plum; (2) to the darker purple of the mulberry; and (3) to the bright clear red of the sweet orange. These characters—viz., alkalinity, bitterness, and the property of precipitation by potassic chromate in a definite crystalline form, the crystals giving the colours detailed—belong to no other substance known save strychnine, and for all purposes sufficiently identify the alkaloid. The same colour is obtained by mixing a drop of sulphuric acid with strychnine and a crystal, or speck, of any one of the following substances:—Ferridcyanide of potash, permanganate of potash, peroxide of lead, peroxide of manganese, and cerous hydroxide.
Potassic permanganate and sulphuric acid is the most delicate, and will detect 0·001 mgrm. of strychnine; cerous hydroxide is, on the other hand, most convenient, for cerous hydroxide is white; all the others have colours of their own. Cerous hydroxide is prepared strychnine; 3 gave 3·811 of the chromate, = 78·77 per cent. of strychnine.—Mohr. by dissolving cerium oxalate in dilute sulphuric acid and precipitating with ammonia, filtering and well washing the precipitate; and the latter may be used while moist, and responds well to 1⁄100 mgrm. of strychnine.
The influence of mixtures on the colour reactions of strychnine have been studied by Flückiger, who states:—
“No strychnine reaction appears with sulphuric acid containing chromic acid (made by dissolving 0·02 grm. of pot. bichromate in 10 c.c. of water, and then adding 30 grms. strong sulphuric acid) when brucine and strychnine mixed in equal parts are submitted to the test; it succeeds, however, in this proportion with sulphuric acid containing potassium permanganate (·02 grm. pot. permanganate in 10 c.c. of water, and 30 grms. of strong sulphuric acid).