URIC ACID, ETC. 399 



insoluble salt is caught on a filter, dried, dissolved in boiling 

 water, mixed with animal charcoal to remove the coloring mat- 

 ter, and filtered while hot ; when the filtrate cools, colorless crys- 

 tals of nitrate of urea are deposited. The precipitate is dissolved 

 in boiling water, and barium carbonate added as long as effer- 

 vescence takes place, barium nitrate and urea being produced. 

 This is evaporated to dryness, and the urea extracted with abso- 

 lute alcohol, which on evaporation leaves crystals of pure urea. 



Estimation. Urea can be estimated volumetrically by the 

 method of Liebig, which depends on the power of mercuric nitrate 

 to give a precipitate with it. The sulphates and phosphates must 

 be first removed by the addition of 40 c.c. of a mixture of 1 

 volume saturated barium nitrate and 2 volumes saturated solu- 

 tion of caustic baryta, to an equal volume of urine. This is 

 filtered, and from the filtrate an amount corresponding to 10 c.c. 

 urine is taken. Into this known volume of urine a standard so- 

 lution of mercuric nitrate (of which 1 c.c. corresponds to 1 cen- 

 tigramme of urea) is dropped until a sample drop of the fluid, 

 mingled on a watch-glass with a drop of concentrated sodium 

 carbonate solution, gives a yellow color, which indicates that some 

 free mercuric nitrate remains. For every cubic centimetre of the 

 standard mercuric solution used there will be 1 centigramme of 

 urea in the sample of urine; a small reduction has to be made 

 for the chlorides, which are present in tolerably constant 

 amount. 



Another simple method consists in mixing together known 

 quantities of urine and sodium hypobromite (NaBrO) with ex- 

 cess of caustic soda. The urea is decomposed in the presence of 

 this salt, and free nitrogen evolved : 



CON 2 H 4 + 3(NaBrO) + 2(NaOH) = SKaBr + Na 2 CO 3 + . 

 3H 2 O+2N. 



The quantity of urea may be determined by ascertaining the 

 volume of nitrogen, which can be measured directly in a grad- 

 uated tube. 



Uric acid, of which the formula is C 5 H 4 N 4 O 3 or C 3 H 2 O :i (NH.- 

 CN) 2 , is present only in extremely small quantities in the normal 



