The ordinary precipitating agents, according to Dragendorff, act as follows:—
Potass bismuth iodide.
- 1 : 2000, a strong orange precipitate.
- 1 : 3000. The drop of the reagent is surrounded with a muddy border.
- 1 : 4000. The drop of the reagent is surrounded with a muddy border.
- 1 : 5000, still perceptible.
- 1 : 6000. The last limit of the reaction.
Phosphomolybdic acid gives a strong yellow precipitate; limit, 1 : 5000.
Potass. mercuric iodide gives a cheesy precipitate; limit, 1 : 1000 in neutral, 1 : 800 in acid, solutions.
Potass. cadmic iodide gives an amorphous precipitate, 1 : 300. The precipitate is soluble in excess of the precipitant. (Nicotine, under similar circumstances, gives a crystalline precipitate.)
Flückiger recommends the following reaction:[343]—“Add to 10 drops of ether in a shallow glass crystallising dish 2 drops of coniine, and cover with filter paper. Set upon the paper a common-sized watch-glass containing bromine water, and invert a beaker over the whole arrangement. Needle-shaped crystals of coniine hydro-bromine soon form in the dish as well as in the watch-glass.” Hydrochloric acid, used in the same way, instead of bromine water, forms with coniine microscopic needles of coniine hydrochlorate; both the hydro-bromide and the hydrochlorate doubly refract light. Nicotine does not respond to this reaction.
[343] Reactions, by F. A. Flückiger, Detroit, 1893.