Additions. 399 



yellow precipitate of oxide of mercury or of a basic 

 mercuric carbonate. 



Second estimation. In the preliminary estimation too 

 niuch mercuric nitrate will probably have been added ; 

 so the process should be repeated, adding at once the 

 amount of mercuric solution found in the preliminary 

 estimation less 1 c,c. After well mixing, a drop is 

 added to a drop of sodium carbonate ; if a yellow 

 colour results, the process must be repeated adding 

 less mercuric nitrate; probably however the mixed 

 drops will be colourless, if so add ■! c.c. mercuric 

 nitrate, mix and test again, and so on adding '1 c.c. at 

 a time until a yellow colour is obtained. 



Correction. Since the sodium chloride in the urine 

 prevents the precipitation of urea by mercuric nitrate 

 (cf. § 5 e) more mercuric nitrate is added to the urine 

 than is required to precipitate the urea in it. The 

 excess thus added is found experimentally to be about 

 2 c.c. for 10 C.C. of urine. Hence 2 c.c. must he deducted 

 from the numher of c.c. of the standard solution actually 



If V be the corrected number of c.c. of mercuric nitrate 

 used, the 10 c.c. of urine taken, contain ■Qlu grams of 

 urea, i.e. the percentage of urea in the urine is the O'v. 



Note. The method as above given is very-rough. For 

 an accurate estimation, the sodium chloride as well as 

 the phosphates must be separated from the urine, the 

 filtered, mixture must .be neutralized, and again neu- 

 tralized with sodium carbonate after each addition of 

 mercuric nitrate. It must also be remembered that 

 the amount of urea found by this method is too high, 



