Benzoic acid in the subliming cell begins to give a cloud at about 77°-80°, and at or near 100° sublimes most rapidly.
Benzoic acid, recovered from an acid solution by shaking out with ether, may be recognised as follows:—To the film left on evaporating off the ether add a drop of H2SO4, and a few crystals of sodic nitrate, and heat gently for a short time; pour the clear liquid into ammonia water, and add a drop of ammonium sulphide. A red-brown colour indicates benzoic acid. The rationale of the test is as follows:—Dinitro-benzoic acid is first formed, and next, by the action of ammonium sulphide, this is converted into the red-brown ammonium diamidobenzoate.—E. Mohler, Bull. Soc. Chem. (3), iii. 414-416.
§ 427. Pyraconitine, C31H41NO10, is anhydro-benzoyl-aconine; it differs from benzoyl-aconine by a molecule of water; picraconitine is obtained by keeping aconitine at its melting-point (188°-190°) for some time, when acetic acid distils over and pyraconitine is left. Pyraconitine is an amorphous varnish, sparingly soluble in water, but readily dissolving in alcohol, chloroform, and ether; it gives a pale yellow precipitate with gold chloride, and forms crystalline salts with hydriodic, hydrobromic, and hydrochloric acids. Pyraconitine readily undergoes hydrolysis by the action of dilute acids, or by potash or soda, or with water in a closed tube; the products are benzoic acid and an alkaloid, to which the name of pyraconine has been given.
§ 428. Pyraconine, C24H37NO9.—This base is anhydro-aconine, the formula differing from aconine by one atom of water. It is amorphous, closely resembling aconine; it is soluble in water and ether; the aqueous solution has a somewhat sweet taste, and is lævorotatory; it combines with acids to form crystalline salts, which are very soluble in water.
§ 429. Aconine, C24H39NO10, m.p. 132°.—Aconine does not crystallise. Its aqueous solution is decidedly alkaline, and, like aconitine, it is lævorotatory, although to a less degree. Its taste is bitter, but causes no tingling sensation. Aconine is very soluble in water or alcohol, and slightly in chloroform, but insoluble in ether or in petroleum ether. It does, however, dissolve, in the presence of aconitine, slightly in ether. The aqueous solutions reduce the salts of gold and silver, and also Fehling’s solution. A solution of aconine gives precipitates with the general alkaloidal reagents; with mercuric chloride it gives a copious yellow precipitate, which darkens on standing.
Aconine hydrochloride, the hydriodide, the hydrobromide, and the sulphate, have all been crystallised; solutions of these salts are lævorotatory.
§ 430. Commercial Aconitine and the Lethal Dose of Aconitine.—Commercial aconitine has in the past varied in appearance from that of a gummy amorphous mass up to a purer kind in white crystals.
Professor Dunstan[470] has recently examined fourteen samples, some of them of considerable age, and only found two samples (one of English, another of German make) which approached in melting-point and crystalline appearance pure aconitine; the one, the English, melted at 186°-187°, and contained about 3 per cent. of benzoyl-aconine; the other, a German specimen, was almost pure; the melting-point was 187·5°. At the present time it is, however, not difficult to obtain fairly pure crystalline aconitine, and to assay it accurately by determining the proportion of acetic and benzoic acids. The physiological action of commercial aconitine is, however, in all cases the same, the difference being in quantitative not qualitative action; in the small doses usually administered, the physiological action depends wholly upon the true aconitine present, the other bases being practically without toxic action. Professor Plugge[471] has made some researches on the fatal dose (for the lower animals) of Petit’s, Merck’s, and Friedländer’s aconitine nitrate, which in 1882 were the purest in commerce. He administered the following doses to the animals mentioned:—
[470] Journ. Chem. Soc. Trans., 1893, 491.