THE ORGANIC CONSTITUENTS OF THE URINE. 243 



solved in a dilute solution of sodium hydrate, when upon the care- 

 ful addition of a dilute solution of copper sulphate a beautiful, 

 purple-red color develops. 



A very delicate test also is the following : 2 c.c. of a concentrated 

 solution of furfurol are treated with 4-6 drops of strong hydro- 

 chloric acid. If to this mixture, which should not present a red 

 color, a small crystal of urea is then added, a deep violet develops 

 in the course of a few minutes. 



Synthetic Formation. As has been mentioned, urea was the first 

 organic substance formed in the animal body which was made syn- 

 thetically in the chemical laboratory. Wohler in 1828 produced the 

 substance artificially by evaporating an aqueous solution of ammo- 

 nium cyanate, when a rearrangement of atoms occurs and urea results : 



X NH 2 



(NH 4 )CNO = C0( 



X NH 2 



Other methods now exist by which urea can also be made syn- 

 thetically, but they are all more or less modifications of the one just 

 described. 



Isolation from the Urine. To isolate urea on a small scale, 50-100 

 c.c. of urine are evaporated to a syrup on a water-bath and ex- 

 tracted with 150 c.c. of strong alcohol by rubbing in a mortar. 

 The alcoholic extract is filtered, the alcohol distilled off, and the 

 syrupy residue treated with concentrated nitric acid in the cold. 

 The urea nitrate which crystallizes out on standing is filtered 

 off with a suction-pump, dissolved in hot water, and the aqueous 

 solution decolorized by gently heating with animal charcoal. The 

 colorless filtrate is then treated with barium carbonate in substance 

 so long as carbon dioxide is evolved, and finally rendered alkaline 

 with barium hydrate solution. The urea is thus liberated according 

 to the equation : 



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



The solution is now evaporated to dryness and the residue extracted 

 with absolute alcohol. On concentrating this extract the urea crys- 

 tallizes out in colorless prisms, which may then be treated as above 

 indicated. 



Quantitative Estimation. In the clinical laboratory the old method 

 of Knop and Hiifner is almost exclusively employed. This is based 

 upon the decomposition of urea with sodium hypobromite in alkaline 

 solution, as already described. The nitrogen which is thus liber- 

 ated is measured and the corresponding amount of urea determined 

 by calculation. 



For scientific purposes, however, this method in any one of its 

 numerous modifications is not sufficiently accurate, as the actual 

 volume of nitrogen which is obtained always falls somewhat short 

 of the theoretical amount. The sodium hypobromite, moreover, 

 also causes a partial decomposition of other nitrogenous constituents 



