712 PHYSIOLOGICAL CHEMISTRY. 



e. By the action of carbonyl chloride, COC1. 2 , on ammonia : 

 COC1 2 + 2NH 3 = : 2HC1 + CO(NH 2 ) 2 . 

 /. By the action of ammonia on ethyl carbonate : 



(C 2 H 5 ) 2 C0 3 + 2NH 3 2C 2 H 5 OH + CO(NH 2 ) 2 . 



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 .HNO 3 ] + BaCO 3 = 2CO(NH 2 ) 2 + Ba(NO 3 ) 2 + CO 2 + H 2 O. 



Experiment 95. 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, dissolve in hot 

 water, and, while the solution boils gently, add small quantities of potassium 

 permanganate until the solution is colorless. To the hot solution add freshly 

 precipitated barium carbonate as long as carbon dioxide escapes. Filter and 

 evaporate the solution to dryness over a water-bath ; boil the mass with alco- 

 hol, which dissolves the urea, but does not act on the barium nitrate. Allow 

 the urea to crystallize from the alcoholic solution. 



Reactions 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, shin- 

 ing, 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. 



Determination of urea. The amount of urea in twenty-four hours 

 is normally from 25 to 35 grammes. The greater part of it is derived 



