ORGANIC CONSTITUENTS. 161 



Organic Constituents. 17 



Urea average percentage 2 p - c - (Blood 0-02 pc -) Quantity in 

 24 hours about 30 g depending on the flesh in the food. 

 Occurs in animal fluids, not in muscle (creatin). 



Properties : 



1. Crystallised slowly, forms prisms ; quickly, long needles. 



2. It is freely soluble in water and absolute alcohol, but insoluble 



in ether and benzene. 



3. On heating dry to 130 C. it melts, giving off NH 3 with 



the formation of biuret as follows : 



2 + NH, 



Urea. Biuret. 



3CO(NH 2 ) 2 - C 3 H 3 N 8 O 3 (cyanuric acid) + 3 NH,. Cyanuric acid 

 solidifies in the tube, and yields on further heating cyanic acid 

 which volatilises, no residue being left if the urea is pure. 



4. It combines with mineral acids, metallic oxides, and salts 

 to form compounds in which the molecule of urea is 

 united to one or more of the reagents. These form pps. 



Recognition of the solid substance. Use commercial urea in 

 crystals. 



1. Place some crystals in a dry tube and heat, they melt 



(1 30 C.) with the odour of NH,. Cool, add a few drops 

 of water and apply the biuret reaction. 



2. Dissolve a few crystals in a drop of water, place portions 



on glass slips. Add to one HNO 3 and to the other a 

 saturated solution of oxalic acid. Each yields a crystalline 

 mass with characteristic crystals of nitrate and oxalate 

 of urea respectively. Examine the crystals. 



