342 PHYSIOLOGICAL CHEMISTRY. 



tion of a baryta solution before titration. PFLUGEE also suggested 

 that the acidity produced by the mercury solution be neutralized 

 with a soda solution. The liquids necessary for the titration are 

 the following: 



1. Mercuric Nitrate Solution. This solution is calculated for a 2$ 

 urea solution, and 20 c. c. of the first should correspond to 10 c. c. of 

 the latter. Each c. c. of the mercury solution corresponds to 

 0.01 grm. urea. As a small excess of HgO is necessary in the 

 urine to make the final reaction (with alkali carbonate or bicarbon- 

 ate) appear, each c. c. of the mercury solution must contain 0.0772 

 instead of 0.0720 grm. HgO. The mercury solution contains there- 

 fore 77.2 grms. HgO in one litre. 



The solution may be prepared from pure mercury or mercuric oxide by 

 dissolving in nitric acid. The solution, freed as completely as possible from an 

 excess of acid, is diluted by the careful addition of water, stirring meanwhile, 

 until it has a specific gravity of 1.10 or a little higher at -|- 20 C. The solu- 

 tion is standardized with a 2% solution of pure urea which has been dried 

 over sulphuric acid, and the operation conducted as will be described later. 

 If the solution is too concentrated, it is corrected by the careful addition of the 

 necessary amount of water, avoiding basic salt, and titrated again. The solu- 

 tion is correct if 19.8 c. c. of it is added at ouce to 10 c. c. of the urea solu- 

 tion and the necessary quantity of normal soda solution (11-1 2 c. c. or more) to 

 completely neutralize the liquid, gives the final reaction when 20 c. c. of the 

 mercury solution have been employed. 



2. Baryta Solution. This consists of 1 vol. barium-nitrate and 

 2 vols. barium-hydrate solution, both saturated at the ordinary 

 temperature. 



3. Normal Soda Solution. This solution contains 53 grms. pure 

 anhydrous carbonate in 1 litre of water. According to PFLUGEB, a 

 solution having a specific gravity of 1.053 is sufficient. The 

 amount of this soda solution necessary to completely neutralize the 

 acid set free during the titration is determined by titrating with ;i 

 pure 2$ urea solution. To facilitate operations a table can be 

 made showing the quantity of soda solution necessary when from 

 10 to 35 c. c. of the mercury solution is used. 



Before the titration the following must be considered. The 

 chlorides of the urine interfere with the titration in that a part of 

 the mercuric nitrate is transformed into mercuric chloride, which 

 does not precipitate the urea. The chlorides of the urine are there- 

 fore removed by a silver-nitrate solution, which also removes any 

 bromine or iodine combinations which may exist in the urine. If 

 the urine contains albumin in noticeable amounts, it must be 

 removed by coagulation and the addition of acetic acid, but care 

 must be taken that the concentration and the volume of the urine 

 is not Changed during these operations. If the urine contains 

 ammonium carbonate in notable quantities, caused by alkaline 



