PROPERTIES AND REACTIONS OF UREA. 687 



plates, with an angle of 82. When crystallizing slowly, larger and 

 thicker rhombic pillars or plates are obtained. This compound is rather 

 easily soluble in pure water, but is considerably less soluble in water 

 containing nitric acid; it may be obtained by treating a concentrated 

 solution of urea with an excess of strong nitric acid free from nitrous 

 acid. On heating this compound it volatilizes without leaving a residue. 



This compound may be employed with advantage in detecting small amounts 

 of urea. A drop of the concentrated solution is placed on a microscope slide and 

 the cover-glass placed upon it; a drop of nitric acid is then placed on the side 

 of the cover-glass and allowed to flow under. The formation of crystals begins 

 where the solution and the nitric acid meet. Alkali nitrates may crystallize 

 very similarly to urea nitrate when they are contaminated with other bodies; 

 therefore, in testing for urea, the crystals must be identified as urea nitrate by 

 heating and by other means. 



UREA OXALATE, 2.CO(NH2)2-H2C204. This compound is more 

 sparingly soluble in water than the nitric-acid compound. It is obtained 

 in rhombic or six-sided prisms or plates on adding a saturated oxalic- 

 acid solution to a concentrated solution of urea. 



Urea also forms combinations with mercuric nitrate in variable 

 proportions. If a very faintly acid mercuric-nitrate solution is added 

 to a 2 per cent solution of urea and the mixture carefully neutralized, 

 a compound is obtained of a constant composition which contains for 

 every 10 parts of urea 72 parts of mercuric oxide. This compound serves 

 as the basis of LIEBIG'S titration method. Urea also combines with 

 salts, forming mostly crystallizable combinations, as, for instance, with 

 sodium chloride, with the chlorides of the heavy metals, etc. An alka- 

 line but not a neutral solution of urea is precipitated by mercuric chloride. 



If urea is dissolved in dilute hydrochloric acid and then an excess of formal- 

 dehyde is added, a thick, white, granular precipitate is obtained which is dif- 

 ficultly soluble and whose composition is somewhat disputed. 1 With phenyl- 

 hydrazine, urea in strong acetic acid gives a colorless crystalline compound of 

 phenylsemicarbazid, CeHeNH.NHiCONHU, which is soluble with difficulty in cold 

 water and melts at 172 C. (JAFF 2 ). 



The method of preparing urea from urine is in the main as follows: 

 Concentrate the urine, which has been faintly acidified with sulphuric 

 acid, at a low temperature, add an excess of nitric acid, at the same time 

 keeping the mixture cool, press the precipitate well, decompose it in 

 water with freshly precipitated barium carbonate, dry on the water- 

 bath, extract the residue with strong alcohol, decolorize when necessary 

 with animal charcoal, and filter while warm. The urea which crystallizes 



*See Tollens and his pupils, Ber. d. deutsch. chem. Gesellsch., 29, 2751; Gold- 

 schmidt, ibid., 29; and Chem. Centralbl., 1897, 1, 33; Thorns, ibid., 2, 144 and 737. 

 2 Zeitschr. f. physiol. Chem., 22. 



