§ 175. ESTIMATWX OF PILOCARPINE, ETC. 185 



The same reagent has been employed hj PoehP for the 

 gravimetric estimation of pilocarpine, but that chemist admits that 

 the results obtained are only apjiroximate. B}* his method 10 

 grams of jaborandi leaves are extracted with 100 cc. of water 

 containing 1 per cent, of hydrochloric acid ; the infusion is 

 precipitated with acetate of lead, the excess of which is removed 

 by hydrochloric acid, and then, after filtration, phosphomolybdic 

 acid is added. The precipitate is collected, washed with water 

 containing a little hydrochloric acid, dried at 100° and weighed. 

 It is said to contain 45 "66 per cent, of pilocarimie. 



In estimating the alkaloid in solutions of the pure substance, 

 phosphomolybdic acid would probably in many cases yield better 

 results than potassio-mercuric iodide ; but there is a certain danger 

 attending its use, and that is the possibility in many cases of 

 ammonia and amidic compounds being precipitated with and 

 calculated as alkaloid. Of pilocarpine in particular it must be 

 observed that, according to Christensen, the composition of the 

 phosphomolybdic acid precipitate, as given hy Poehl, requires 

 revision. 



Phos2)homoIijhdate of quinine (dried below 70'') contains, according 

 to Prescott, 2 7* 3 per cent, of quinine. 



For cases in which ])hos])hotnngstic acid may be employed see 

 §' 177. 



Attempts have also been made to estimate alkaloids by means 

 of tannic acid,- by either drying the precipitate produced or 

 liberating the alkaloid from it ^vith oxide of lead or other base, 

 drying and weighing. My objection to the former of these 

 two methods is that the tannates of the alkaloids are scarcely 

 ever constant in their composition. The latter might be adopted in 

 certain cases provided that the precipitated tannate is sufficiently 

 sparingly soluble, and that the alkaloid itself is not attacked, 

 as curarine is, by the oxide of lead used to decompose its tannate 

 (§ 64). 



^ 'Unters, d. Blatter des Pilocari^us officinalis,' St. Petersburg, 1S77 (Year- 

 book of Pharm. 28, 141, 1881). See also Harnack and Meyer, Annal. d. 

 Chem, und Pharm, cciv. 67, 1880 (Pharm. Journ. and Trans. [3], xi. 551, 587» 

 608) ; and Christensen, Pharm. Zeitschr. f. Faissland, xx. ISSl (Pharm. Journ. 

 and Trans. [3], xii. 400). 



- Compare, for instance, Lefort, .Journ. de Pharm, et de Chimie, ix. 117, 

 241, 1869 (Pharm. Journ. and Trans. [3], ii. lü-29 ; iii. 63). See also my 

 ' Werthljestimuiunf>-, ' 40. 



