THE ORGANIC CONSTITUENTS OF THE URINE. 241 



Properties of Urea. Urea crystallizes in two forms, viz., in 

 long, fine white needles if rapidly formed, or in long, colorless 

 rhombic prisms when allowed to crystallize more slowly from its 

 solutions. 



It melts at 130 to 132 C., but is probably decomposed already 

 at a temperature of 100 C. It is readily soluble in water and 

 alcohol, but is insoluble in anhydrous ether, chloroform, and benzol. 

 As the substance is an acid amide, its solutions present a neutral 

 reaction. 



In accordance with its character as an unsaturated amide of 

 carbonic acid, however, it combines with acids to form crystalline, 

 salt-like compounds. The most important of these are the nitrate 

 and the oxalate. 



Urea nitrate, CO(NH 2 ) 2 .HNO 3 , crystallizes in two forms, viz., in 

 delicate rhombic, horizontal platelets, which are commonly arranged 

 overlapping in a shingle-like manner when rapidly formed, or as 

 thicker rhombic columns or plates when allowed to crystallize more 

 slowly. 



Urea nitrate is readily soluble in distilled water, but dissolves 

 with difficulty if this is acidulated with nitric acid, and also in 

 alcohol. Its formation is frequently observed when urine contain- 

 ing much urea is examined for albumin in the cold with nitric 

 acid. On standing, the nitrate may then separate out in crystalline 

 form. On heating, the substance is decomposed without leaving a 

 residue. 



Urea oxalate, CO(NH 2 ) 2 .C 2 H 2 O 4 , crystallizes in rhombic plates, or 

 hexagonal prisms, and is less soluble in water than the nitrate ; in 

 alcohol and in dilute solutions of oxalic acid it is nearly insoluble. 

 The substance is obtained in crystalline form on adding a saturated 

 solution of oxalic acid to a concentrated solution of urea. 



Urea also combines with various neutral salts, such as sodium 

 chloride and ammonium chloride, and also with the nitrates of 

 sodium and the oxides of silver and mercury, to form double salts. 

 With mercuric nitrate three different compounds result, according 

 to the concentration of the two solutions, viz., CO(NH 2 ) 2 .Hg 2 (NO 3 ) 4 , 

 CO(NH 2 ) 2 .Hg 3 (N0 3 ) 6 , and [CO(NH 2 ) 2 ] 2 .Hg(NO 3 ) 2 + 3HgO. The 

 latter compound is of special interest, as Liebig's quantitative esti- 

 mation of urea, which was formerly much employed, was based 

 upon its formation. It results when a 2 per cent, solution of urea 

 is treated with a feebly acid solution of mercuric nitrate, and the 

 mixture is subsequently neutralized. 



Mercuric chloride precipitates urea in alkaline, but not in neutral 

 solutions. 



Very important, further, is the behavior of urea toward sodium 

 hypochlorite or hypobromite, as the most usual method of esti- 

 mating urea in the clinical laboratory is based upon the reaction 

 which here takes place. This reaction may be represented by the 

 equation : 



16 



