654 URINE. 



UREA OXALATE, 2.CO(NH 2 )2.H 2 C2O4. 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 cry stalliz able 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 difficultly 

 soluble and whose composition is somewhat disputed. 1 With phenylhydrazine, 

 urea in strong acetic acid gives a colorless crystalline compound of phenyl- 

 semicarbazid, C 6 H 6 NH.NH.CONH 2 , which is soluble with difficulty in cold water 

 and melts at 172 C. (JAFFE 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 anima) charcoal, and filter while warm. The urea which crystallizes 

 on cooling is purified by recrystallization from warm alcohol. A fur- 

 ther quantity of urea may be obtained from the mother-liquor by con- 

 centration. The urea is purified from contaminating mineral bodies 

 by redissolving in alcohol-ether. If it is only necessary to detect the 

 presence of urea in urine, it is sufficient to concentrate a little of the 

 urine on a watch-glass and, after cooling, treat it with an excess of nitric 

 acid. In this way we obtain crystals of urea nitrate. 



Quantitative Estimation of the Total Nitrogen and Urea in Urine. 

 Among the various methods proposed for the estimation of the total 

 nitrogen, that suggested by KJELDAHL is to be recommended. LIEBIG'S 

 method for the estimation of urea is really a method for determining 

 the total nitrogen, but as it is very seldom used now we can refer to 

 larger works in regard to details. 



KJELDAHL'S method consists in transforming all the nitrogen of the 

 organic substances into ammonia by heating with a sufficiently con- 

 centrated sulphuric acid. The ammonia is distilled off after super- 



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

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

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



