44 HUMAN PHYSIOLOGY 



Compounds of nrca. With many acids and bases, urea 

 forms characteristic compounds. The acids unite with the 

 ammonia group, in which the nitrogen becomes quinquiva- 

 lent. The most important compounds are : 



(a) Urea nitrate, CO(NH 2 ) 2 .HNO. r It crystallizes in 

 smooth, hexagonal, colorless platelets, soluble in pure water 

 but soluble with difficulty in water containing nitric acid. 

 The crystals are obtained by adding an excess of strong 

 nitric acid to a concentrated solution of urea. The com- 

 pound serves for the detection and isolation of urea. 



(b) Urea oxalate, [CO(NH 2 )J 2 .C 2 H 2 O 4 . 



(c) A' white precipitate is formed when a urea solution is- 

 mixed with a solution of mercuric nitrate. The proportion 

 in which urea and mercuric nitrate unite varies with the con- 

 centration of the urea and the mercuric nitrate used. Upon 

 the precipitation of urea by mercuric nitrate depends Liebig's 

 titration method for estimating urea. 



Synthesis of urea. Urea is formed: 



1 . By heating ammonium cyanate : 



NH 4 .O.CN = CO(NH 2 ) 2 . (Wohler, 1828.) 



2. By heating ammonium carbonate with metallic sodium, 

 water being split off. 



3. By passing an alternating electrical current through a 

 solution of ammonium carbamate, NH 2 .COONH 4 , water 

 being split off from the ammonium carbamate. 



4. From carbonylchloride and ammonia: 



COC1 2 + 2NH, = CO(NH 2 ) 2 + 2HC1. 



5. From ethyl carbonate and ammonia: 

 CO,(C 2 H.) 2 + 2NH, = 2(C 2 H..OH) + CO(NH 2 ) 2 . 



Presence and formation of nrca in the animal body Urea 

 forms most of the solids of the urine of mammals. It is 

 present in small quantities in the blood, in all tissue fluids, 

 and in many organs of the body. 



Urea is the most important nitrogenous end-product of 



