408 CHEMISTRY OF THE LEUCOMAJNS. 



silver salts remaining in solution in the nitric acid, the " xanthin 

 fraction," consists of xanthin, paraxanthin, and heteroxanthin. 



The urine, acidulated with hydrochloric acid, is precipitated with 

 phosphotungstic acid; the precipitate is decomposed by warming 

 with baryta, filtered, and the filtrate is freed from barium by the 

 cautious addition of sulphuric acid. The solution is then made alka- 

 line with ammonium hydrate, any traces of phosphates that appear 

 are filtered off. Or, the urine may be rendered directly alkaline 

 with ammonium hydrate and after standing 24 hours the precipi- 

 tated phosphates removed by filtration. The filtrate finally is pre- 

 cipitated by the addition of ammoniacal silver nitrate (0.5 g. per 

 liter). The precipitate which forms consists of the silver compounds 

 of the xanthin bodies, and is purified by dissolving in boiling nitric 

 acid of 1.1 sp. g. The filtrate, on cooling, yields a precipitate of 

 the silver nitrate compounds of adenin, hypoxanthin, and guanin. 

 The xanthin compound remains in solution. The silver salts of the 

 three bases mentioned are decomposed with dilute hydrochloric, or 

 with hydrogen or sodium or ammonium sulphid, and the acid solu- 

 tion of the three bases is then heated on the water-bath with excess 

 of ammonia. Guanin is thus thrown out of solution. In the am- 

 moniacal filtrate the adenin is separated from the hypoxanthin and 

 estimated by picric acid (see p. 411). The hypoxanthin is esti- 

 mated as hypoxanthin silver picrate. Instead of the separation of 

 guanin from the other bases by ammonia Wulff's metaphosphate 

 method may be employed (p. 385). The adenin and hypoxanthin 

 are then precipitated from the filtrate by ammoniacal silver solution. 

 The silver salts decomposed with hydrochloric acid, and in the 

 filtrate the adenin can be estimated as the picrate ; the hypoxanthin 

 as the hypoxanthin silver picrate. 



A second method for the extraction of the xanthin bases is based 

 upon Drechsel's reaction, namely, that they are precipitated by a cop- 

 per solution in the presence of a reducing agent (see page 353). The 

 precipitation is so complete that the filtrate does not react with am- 

 moniacal silver nitrate. The reliability of the process has been 

 quantitatively tested by Balke, who found the yield of xanthin bases 

 from meat extract to be slightly greater by the copper method over 

 Neubauer's silver nitrate process. In addition to cheapness the 

 method possesses an advantage in the less bulky precipitates. 



As reagents Balke employed Fehling's solution and hydroxylamin 

 hydrochlorid, whereas Kriiger made use of copper sulphate (13 per 

 cent.) and sodium bisulphite (1-2). The latter solutions are ob- 

 viously preferable. 



The urine or other liquid to be examined is first freed from any 

 albumin that may be present. It is then boiled and the copper and 

 bisulphite solutions are added. Barium chlorid may be added to 

 facilitate the settling of the precipitate, which after standing several 



