106 UREA. HIPPURIC ACID. AMINOACIDS 



of nitrogenous food. If the ingested material has been in 

 the form of protein already advanced in catabolism the maxi- 

 mum is reached more quickly and the greatest urea excretion 

 is to be noted within one to two hours. 18 



Quantitative Estimation of Urea. The very great 

 physiological importance of urea is apparently sufficient 

 reason for the fact that there has been no dearth of 

 effort to improve the methods for its quantitative de- 

 termination. 19 The old method of Liebig by titration with 

 mercuric nitrate continues to appear here and there in litera- 

 ture; but has become practically unimportant, and efforts 

 to reinstate the method have attained but little success. 20 

 The most modern methods as a rule are based upon the prin- 

 ciple of transforming the urea into ammonia by hydrolytic 

 agents and distilling the ammonia. Other nitrogenous con- 

 stituents present, especially those of basic nature, may be 

 separated by phosphotungstic acid (Pfluger-Bleibtreu- 

 Schondorf method) or by phosphomolybdic acid (Haskin's 

 method). 21 Some prefer to at least partially separate the 

 urea from other constituents by the Morner-Sjoquist 

 method (precipitation by alcohol and ether in presence of 

 baryta) . Hydrolysis of the urea is accomplished by heating 

 with magnesium chloride and hydrochloric acid (Folin's 

 method), 22 with lithium chloride and hydrochloric acid 

 (Saint Martin), 23 with hydrochloric acid in sealed tubes 

 (Salaskin and Zaleski), with sulphuric or hydrochloric acid 



"A. Stauber (E. Freund's Lab., Vienna), Biochem. Zeitschr., 25, 187, 1910. 



** Literature upon the Quantitative Estimation of Urea : P. Rona, Handb. d. 

 biochem. Arbeitsmethoden, 3', 774-782, 1910; 5', 295, 1911; Neubauer-Huppert, 

 Harnanalyse, llth ed., article by W. Wiechowski, I, 560-576, 1910; C. Neuberg, 

 Der Ham, article by A. C. Anderson, 1, 631-641, 1911; Ch. Sallerin, Journ. de 

 Physiol., 1903, No. 2. 



20 B. Glaszmann (Odessa), Ber. d. Deutsch. chem. Ges., 39, 705, 1906. 



* H. D. Haskins, Jour, of Biol. Chem., 2, 243, 1906. 



O. Folin, Zeitschr. f. physiol. Chem., 36, 333, 1902; 37, 548, 1903; E. P. 

 Cathcart, Jour, of Physiol., 35, Proc. viii, 1906. 



88 L. G. Saint Martin, C. R. Soc. de Biol., 58, 89, 1905. 



