THE URINE. 341 



which contains for every 10 parts of urea 7.2 parts mercuric oxide. 

 This compound serves as the basis of LIEBIG'S titration method. 

 Urea combines also with salts, forming mostly crystal lizable combi- 

 nations, as, for instance, with sodium chloride, with the chlorides of 

 the heavy metals, etc. An alkaline but not a neutral solution of 

 urea is precipitated with mercuric chloride. 



The method of preparing urea from urine is chiefly as follows : 

 Concentrate the urine, which has been faintly acidified with sul- 

 phuric 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 on cooling is purified by 

 recrystallization from warm alcohol. A further quantity of urea 

 may be obtained from the mother-liquor by concentration. 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 with an excess of nitric acid. 

 In this way we obtain crystals of urea nitrate. 



Quantitative Estimation of Urea in urine. The methods 

 suggested for this purpose are those of LIEBIG, by titration, of 

 HEINTZ and EAGSKY, also that of KJELDAHL, by which the total 

 nitrogen is determined, and those of BUNSEN and KNOP-HUFNER, 

 where urea is intended to be determined as such. Among these 

 methods, that of LIEBIG, which is perhaps the one most frequently 

 employed by physicians, will here be carefully explained. In regard 

 to the others, whose chief points only will be spoken of here, the 

 student is referred to other text-books. 



LIEBIG'S METHOD is based upon the fact that a dilute solution 

 of mercuric nitrate under proper conditions precipitates all the urea 

 forming a compound of constant composition. As indicator, a 

 soda solution or a thin paste of sodium bicarbonate is used. An 

 excess of mercuric nitrate produces herewith a yellow or yellowish- 

 brown combination, while the combination of urea and mercury is 

 white. PFLUGER has given full particulars of this method; there- 

 fore we will describe PFLUGER'S modification of LIEBIG'S method. 



As phosphoric acid is also precipitated by the mercuric- 

 nitrate solution, this must be removed from the urine by the addi- 



