SEPARATION OF THE PVBIN BASES. 409 



hours is filtered off and repeatedly washed with warm water (60°). 

 The precipitate is then suspended in acidulated water and decom- 

 posed with hydrogen sulphid. To destroy the uric acid which may 

 be present the solution is acidulated with acetic acid and boiled with 

 manganese dioxid, after which the dissolved manganese is precipi- 

 tated by boiling with ammonium carbonate and hydrate. The purin 

 bases may now be reprecipitated with copper sulphate and bisulphite 

 solutions. 



All the xanthin compounds containing an imido group capable of 

 substitution, with the exception of theobromin, are precipitated. 

 Caffein and dimethyl hypoxanthin contain no imido group, and are 

 therefore not precipitated. Neither are urea, allantoin, amido acids, 

 pepton, albumose, creatin and creatinin thrown down. The more 

 imido groups present in a compound the less soluble is the precipitate. 



Kriiger and Wulff in 1894 applied the copper method for the 

 determination of the relative amounts of purin bases and uric acid 

 in urines (see Novy's Physiological Chemistry). The ratio of the 

 nitrogen of the former to that of the latter is of little value when it 

 is remembered that the greater part of the purin bases in the urine 

 are derived from the caffein and similar bodies in the food and have 

 nothing to do with cell metabolism. 



The Neubauer method as ordinarily employed does not yield as 

 satisfactory results as may be expected. As pointed out by Kriiger 

 and Salomon the use of nitric acid would cause an oxidation of 

 carnin to hypoxanthin. Moreover, any nitrous acid present would 

 convert more or less completely the guanin into xanthin and the 

 adenin into hypoxanthin. This change as Kriiger has shown is not 

 prevented by the addition of even a large excess of urea. They thus 

 explain their non-detection of carnin and guanin in the urine. A 

 more serious objection was pointed out by them in connection with 

 the separation of the purin bases into the two fractions by means of 

 boiling nitric acidj The supposition that the silver salts of the 

 xanthin fraction are easily soluble in the nitric acid is not correct. 

 Thus, the silver salt of heteroxanthin requires 2,820 parts of nitric 

 acid (1.1 sp. gr.) for solution. It follows therefore that the hypo- 

 xanthin fraction as ordinarily obtained always contains considerable 

 amounts of the xanthin fraction. 



In the course of their exhaustive investigation upon the urinary 

 bases Kriiger and Salomon ' recognized these defects and devised the 

 following method of separation : 



The purin bodies may be precipitated either by means of ammon- 

 iacal silver solution or by means of copper sulphate and sodium 

 bisulphite as already described. In the former case the washed pre- 

 cipitate is transferred to a round bottom fiask and decomposed on a 



•Zeife. phyml. Ghent., 26, 373 (1898). 



