URINE. 91 



ORGANIC COMPOUNDS. 

 NH 2 



UREA, OC< 



X NH 2 



Urea or carbamide is present in the urine in about a 

 2 per cent solution or 30 grms. for 24 hours. It is very 

 soluble in water and alcohol and crystallizes in long rhombic 

 prisms. It possesses weakly basic properties and forms com- 

 binations with acids analogous to double salts. When 

 heated gently, urea yields biuret. The Micrococcus urece 

 splits it into NH 3 and C0 2 . 



Prepare some urea from the urine as follows-: 



Evaporate one-half a liter of urine to syrupy consistency 

 and exhaust the residue with hot alcohol. Filter off the 

 latter and evaporate the nitrate to dryness. This residue 

 is extracted on the water-bath with successive portions of 

 pure acetone, which must be filtered hot. The mixed acetone 

 filtrates are then evaporated slowly to a small volume and 

 allowed to cool. The crystals which separate out may be 

 filtered off and washed with cold acetone. 



Make use of these urea crystals for the following reac- 

 tions : 



(a) Dissolve some urea in a few c.c. 01 water. Place in 

 each of two watch-glasses one drop of the solution. To one 

 add one drop of dilute HNO 3 and to the other a drop of dilute 

 oxalic acid solution. Allow them to stand and then examine 

 the crystals under the microscope. What are these com- 

 pounds? 



(6) Warm carefully a few crystals of urea in a dry test- 

 tube. The substance melts, giving off NH 3 . Continue heat- 



