412 CSEMISTBT OF THE LEUCOMAINS. 



composed in the cold with baryta water, treated with carbonic acid 

 and finally the liquid is warmed on the water-bath and filtered hot. 

 The filtrate is then precipitated with copper sulphate and sodium 

 bisulphite to remove the ordinary purin bases. The filtrate from the 

 copper precipitate is treated with hydrogen sulphid, the resulting 

 clear solution is evaporated and the residue extracted with chloro- 

 form. This takes up the cafiein and theobromin. To separate 

 these the chloroform solution is evaporated and the residue is taken 

 up with water, and the solution treated with silver nitrate and a 

 slight excess of ammonia. The ammonia is then expelled by boil- 

 ing and the solution on cooling yields a deposit of the theobromin 

 silver compound. The filtrate from this deposit is acidulated with 

 hydrochloric acid and extracted with chloroform, which on evapo- 

 ration yields caff em. 



The 3-methyl xanthin is separated from the purin bases as the in- 

 soluble barium salt. Theophyllin is separated from paraxanthin by 

 recrystallizing the silver nitrate compounds from nitric acid (Kriiger).* 



Separation of uric acid from xanthin bases. 



In the precipitation of uric acid by the silver method the xanthin 

 bases if present are likewise thrown down. However, as adenin and 

 hypoxanthin are readily soluble in acids and alkalis, they are easily 

 separated after decomposition of the silver precipitate. Xanthin 

 and guanin are less soluble, and hence may render the uric acid 

 impure especially when this is isolated from animal organs. Wulff 

 detected the presence of xanthin in a uric acid precipitate by destroy- 

 ing the uric acid with hot dilute nitric acid — the xanthin remaining 

 unchanged. The substance is heated on the water-bath with dilute 

 nitric acid (100 parts water and 5 parts nitric acid, 1.14 sp. g.), and 

 after gas ceases to be given off it is boiled for a short time, rendered 

 ammoniacal, and if xanthin is present it gives a precipitate with 

 silver nitrate. The precipitate can be collected on a weighed filter, 

 dried at 120°, and weighed as C^H^N^Oj.AgjO ; or from the weight 

 of silver after ignition the amount of xanthin may be calculated. If 

 the amount of xanthin is large, it can be weighed directly. For this 

 purpose the solution is rendered slightly ammoniacal, after the 

 oxidation with nitric acid, and then is treated with acetic acid and 

 an equal volume of alcohol. After twelve hours filter, wash with 

 dilute alcohol, dry at 110°, and weigh. 



The destruction of uric acid by nitric acid and by manganese 

 dioxid has been already referred to. Horbaczewski's method for the 

 separation of uric acid from xanthin is as follows : The mixture of 

 the two substances is dissolved with aid of gentle heat in a platinum 

 dish in concentrated sulphuric acid (2 c.c. for 0.1 g. substance). 

 1 BeridKU, 32, 2681, 2823, 3836. 



