160 UREA. 



Conversion into carbonic acid and ammonia by heating in a sealed 

 tube with an ammoniacal solution of barium chloride, and deter- 

 mination of the weight of barium carbonate obtained. (Bunsen.) 



Although simple in principle, the above methods, and especially the 

 first, require the careful observance of certain precautions to ensure 

 accuracy. The needful precautions have recently been most assiduously 

 investigated, more particularly by Pfliiger and his pupils, and of these 

 and of the application of the methods a full account is given in 

 Neubauer and Yogel's exhaustive work Die A nalyse des Harns. 



The determination of the total nitrogen in urine is also of. great 

 importance and is now usually carried out by Kjeldahl's method 1 . 

 This consists in converting all the nitrogen of a measured portion of 

 urine into ammonia by boiling with fuming sulphuric acid and the 

 subsequent addition of potassium permanganate. The ammonia is then 

 expelled from the acid solution by distillation with an excess of caustic 

 soda or potash, the ammonia being received into a measured volume of 

 standardised acid, whose diminution of acidity due to the absorption 

 of ammonia is finally determined by titration with standard alkali. 



The synthesis of urea by molecular transformation of ammonium 

 cyanate, indicates an undoubtedly close relationship of urea to cyanic 

 acid, and there are other reactions which enforce the same idea. Thus 

 by the union of water with cyanamide, which is readily effected by 

 treatment with 50 p.c. sulphuric acid, urea is obtained: CN.NH 2 

 -f- H 2 O = (NH 2 ) 2 CO. It is further stated that when potassium cyanate 

 and acid potassium tartrate are dissolved in water and the mixture is 

 kept for some time, a not inconsiderable amount of urea is formed 

 along with some carbonic acid 2 , thus affording experimental support 

 of Salkowski's view 3 that urea might arise in the body from the 

 union of two molecules of cyanic acid and one of water : CO . NH 

 + CO . NH + H 2 O = (NH 2 ) 2 CO + CO 2 . The final formation of cyanuric 

 acid (CO . NH) 3 by the action of heat on dry urea is further evidence 

 in the same direction. On the other hand there are a number of 

 reactions resulting in the production of urea, which leave but little 

 doubt that urea, while closely related to cyanic acid, is truly the amide 

 of carbonic or carbamic acid. Thus, by the action of ammonia on 

 phosgene gas : COC1 2 + 2 NH 3 = CO (KH 2 ) 2 + 2 HC1 : of ammonia on 

 diethyl-carbonate: CO . (C 2 H 5 O) 2 + 2 NH 3 =CO (NH 2 ) 2 4- 2 C 2 H 5 . OH : 



1 Zt.f. anal. Chem. Bd. xxn. (1883), S. 366. 



2 Hoppe-Seyler, Physiol. Chemie, S. 809. 



3 Zt. f. physiol. Chem. Bd. i. (1877), S. 41. 



