It gives a precipitate with iodine trichloride, and has therefore probably a pyridine nucleus, it may be an acid anilide.[452] It gives the same colours as strychnine with sulphuric acid and potassic permanganate or potassic chromate; it causes in frogs tetanus, but the dose has to be much larger than that of strychnine. The duration of life in doses of 15 mgrms. may extend to five days, and frogs may even recover after 50 mgrms.
[452] Julius Tafel (Ber., 1890, 412) has shown that the colour reactions with H2SO4 and oxidising agents are the characteristic tests of an acid anilide.
The distinction between strychnine and hypaphorine is therefore easy; besides it will not occur in a chloroform extract, and it will not give a precipitate with potassic chromate.
§ 400. Quantitative Estimation of Strychnine.—The best process of estimating the proportion of each alkaloid in a mixture of strychnine and brucine, is to precipitate them as picrates, and to destroy the brucine picrate by nitric acid after obtaining the combined weight of the mixed picrates; then to weigh the undestroyed strychnine picrate.
To carry out the process, the solution of the mixed alkaloids must be as neutral as possible. A saturated solution of picric acid is added drop by drop to complete precipitation. A filter paper is dried and weighed, and the precipitate collected on to this filter paper; the precipitate is washed with cold water, dried at 105°, and weighed. This weight gives the combined weight of both strychnine and brucine picrates.
The precipitate is now detached from the filter, washed into a small flask, and heated on the water-bath for some time with nitric acid diluted to 1·056 gravity (about 11 per cent. HNO3). This process destroys the brucine picrate, but leaves the strychnine picrate untouched. The acid liquid is now neutralised with ammonia or soda, and a trace of acetic acid added; the precipitate of strychnine picrate is now collected and weighed. The weight of this subtracted from the first weight, of course, gives that of the brucine picrate.
One part of strychnine picrate is equal to 0·5932 strychnine; and one part of brucine picrate is equal to 0·6324 brucine.
From the strychnine picrate the picric acid may be recovered and weighed by dissolving the picrate in a mineral acid and shaking out with ether; from the acid liquid thus deprived of picric acid the alkaloid may be separated by alkalising with ammonia and shaking out with chloroform.