URINE AND ITS NORMAL CONSTITUENTS. 455 



Urea may be obtained from urine by evaporating it to the consist- 

 ence of a syrup and mixing the cooled residue with an equal volume 

 of nitric acid, when crystals of urea nitrate, CO(NH 2 ) 2 .HNO 3 , form, 

 which may be decomposed by barium carbonate into urea and barium 

 nitrate : 



2[CO(NH 2 ) 2 .HN0 3 ] + BaC0 3 == CO(NH 2 ) 2 + Ba(NO 3 ) 2 + CO 2 + H 2 O. 



Experiment 69. Evaporate about 200 c.c. of urine to a syrupy consistence, 

 allow to cool, place the vessel containing the syrup in ice and add slowly with 

 stirring a volume of nitric acid equal to that of the evaporated urine. Set 

 aside for twenty-four hours, collect the crystalline mass of urea nitrate on a 

 filter, wash with very little cold water, allow to drain well and dissolve in hot 

 water. (If much colored, shake the solution with animal charcoal and filter. ) 

 To the hot solution add freshly precipitated barium carbonate as long as car- 

 bon dioxide escapes. Filter and evaporate the solution to dryness over a water- 

 bath ; boil the mass with alcohol, which dissolves the urea, but does not act on 

 the barium nitrate. Allow the urea to crystallize from the alcoholic solution. 



Reactions and determination of urea. There are no very 

 characteristic reactions 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 evaporated. 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 C. 

 (320 F.) until no more vapors of ammonia are evolved, leaves a 

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

 drops of potassium hydroxide solution and a drop of cupric sulphate 

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

 violet color. 



The quantitative estimation of urea in urine may be effected by vari- 

 ous 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 

 alkali hypobromites into carbon dioxide, water, and nitrogen : 

 CO(NH 2 ) 2 + 3(NaBrO) = 3NaBr + CO 2 + 2H 2 O + 2N. 



The liberated nitrogen is collected, and from its volume its weight 

 and that of the urea are calculated. 



