376 



PHYSIOLOGICAL CHEMISTRY 



what evaporated alcoholic solution yields crystals of urea, which 

 are generally more or less colored by urinary pigments. 



Determination of urea. There are no very characteristic reac- 

 tions by which urea can be well recognized. From organic 

 mixtures it is separated by digesting them with from 3 to 4 

 volumes of alcohol in the cold ; the filtered liquid is evaporated 

 to dryness and extracted with alcohol, which again is evap- 

 orated. The dry residue may be tested for urea as follows : 



1. Dissolved in a few drops of water, the addition of an 

 equal quantity of colorless nitric acid causes the formation of 

 white, shining, crystalline plates or prisms of urea nitrate. 



2. If a strong solution of oxalic acid is added, instead of 

 nitric acid, rhombic plates of urea oxalate form. 



3. The residue (or urea) heated in a test-tube to about 160, 

 until no more vapors of ammonia are evolved, leaves a substance 

 termed biuret, C 2 H 6 N 3 2 , which upon the addition of a few drops 

 of potassium hydrate solution and a drop of cupric sulphate 

 solution, causes the solution of the cupric hydrate with a reddish- 

 violet color. 



The quantitative estimation of urea in urine may be effected by 

 various methods, of- which but one will be mentioned, because 

 it requires less time and less skill in manipulation than most 

 other methods. This determination is based upon the fact that 

 urea is decomposed by alkaline hypobromites into carbon di- 

 oxide, water, and nitrogen : 



CO(NH 2 ) 2 



Urea. 



3(NaBrO) 



Sodium 

 hypobromite. 



3NaBr 



Sodium 

 bromide. 



CO 2 



Carbon 

 dioxide. 



2H 2 O 



Water. 



2N. 



Nitrogen. 



The liberated nitrogen is collected, and from its volume its 

 weight and that of the urea are calculated. 



Practically the operation is conducted as follows: 100 grams 

 of sodium hydrate are dissolved in 250 c.c. of water, and to 

 this cooled solution are added 25 c.c. of pure bromine, when 

 sodium hypobromite is formed, leaving, however, an excess 

 (over one-half) of the sodium hydrate in an unaltered condition. 

 (The solution easily decomposes, and should, therefore, be 

 freshly prepared for analysis.) 



The apparatus required (Fig. 16) consists, in its most simple 



