344 PHYSIOLOGICAL CHEMISTRY. 



color, the addition of mercury solution is continued by adding ^ 

 and then -f^ c. c., and testing after each addition in the following 

 way : A drop of the mixture is placed on a glass plate with a dark 

 background beside a small drop of the bicarbonate paste. If the 

 color after stirring the two drops together is still white after a few 

 seconds, then more mercury solution must be added; if, on the con* 

 trary, it is yellowish, then if not too much mercury solution has 

 been added by inattention the result to T V c. c. has been found. 

 By this approximate determination, which is sufficient in many 

 cases, we have learned the minimum amount of mercury solution 

 necessary to add to the quantity of urine in question, and we now 

 proceed to the final determination. 



A second quantity of the filtrate, corresponding to 10 c. c. of 

 the original urine, is filtered, and the same quantity of mercury 

 solution added at one time as was found necessary to produce the 

 final reaction, and immediately after the corresponding amount of 

 soda solution, which must not indicate the end of the reaction. Then 

 add the mercury solution in -^ c. c. without neutralizing witli soda, 

 until a drop taken out and mixed with the soda solution gives a yellow 

 coloration. If this final reaction is obtained after the addition of 

 0.1-0.2 c. c., then the titration may be considered as finished. If, 

 on the contrary, a larger quantity is necessary, the addition of the 

 mercury solution must be continued until a final reaction is ob- 

 tained with simple carbonate, and the titration repeated again, add- 

 ing the quantity of mercury solution used in the previous test at 

 one time, and also adding the corresponding amount of soda solu- 

 tion. If we obtain the end reaction by the addition of ^ c. c., we 

 may consider the titration as finished. 



If in each titration a quantity of filtrate containing urine and 

 baryta corresponding to 10 c. c. of the primitive urine is used, then 

 the calculations are very simple, since 1 c. c. of mercuric-nitrate 

 solution corresponds to 0.01 grm. of urea. As the mercury solu- 

 tion is made for a 2$ urea solution, the filtrate of urine and baryta 

 being generally deficient in urea (if the quantity of urea is above 

 2$, it is easy to avoid any mistake by diluting the urine at the 

 beginning of the operation), a mistake occurs here which can be cor- 

 rected in the following way, according to PFLUGER : To the measured 

 volume of the filtrate from the urine (the filtrate with baryta after 

 neutralization with nitric acid, precipitation with silver nitrate and 

 filtration) the quantity of normal soda solution employed is added, 

 and from this sum the volume of mercury solution used is sub- 

 tracted. The remainder is then multiplied by 0.08, and the prod- 

 uct subtracted from the number of c. c. of mercury solution used. 

 For example, if the filtrate (urine and baryta + nitric acid -j- silver 

 nitrate) measured 25.8 c. c., and the number of c. c. of soda solu- 

 tion used in the titration, 13.8 c. c., and the mercury solution, 20.5 



