182 QUANTITATIVE ANALYSIS. 



The first determination is only approximate; it must be 

 repeated several times. It is very important, as Pfliiger has 

 shown, to add at once, as nearty as possible, the number of 

 cubic centimeters of the mercury solution which are necessary 

 to complete the reaction. The mean of the several deter- 

 minations is taken as correct. As a rule, the mercury solu- 

 tion is too strong and must be diluted. The amount of 

 water necessary to be added (x) is calculated from the 

 proportion 



a:20 a = v:x-, 



i?(20-a) 

 X= a ' 



in which v is the volume of the mercury solution to be diluted 

 and a the number of cubic centimeters used. Twenty cubic 

 centimeters of the mercury solution will then correspond to 

 0.2 g. of urea. 



(c) Determination of Urea in Urine. 



As the urine always contains phosphoric acid and as 

 phosphoric acid is also precipitated by the mercury solution, 

 it is necessary in making a determination of urea in 

 urine by this method to remove the phosphoric acid. This 

 is done by mixing 50 cc. of the urine, accurately meas- 

 ured, with 25 cc. of Liebig's baryta mixture (2 volumes of 

 baryta-water and 1 volume of barium nitrate solution) and 

 filtering through a dry filter into a dry vessel. Measure off 

 15 cc. of the filtrate, which correspond to 10 cc. of the 

 urine. The titration is performed in the same way as with 

 the urea solution. In regard to the addition of the mercury 

 solution, the specific gravity serves as a guide in the case of 

 normal urines. The number of cubic centimeters of the 

 mercury solution to be added at first is the same as the last 

 two figures of the specific gravity (10 cc. if the specific grav- 



