THE URINE. 205 



XI. DETERMINATION OF HYDROCHLORIC ACID. 



Hydrochloric acid is usually expressed in the analyses as 

 sodium chloride. 



Mohr's Titration Method. Principle: If potassium chro- 

 mate and then silver nitrate solutions be added to sodium 

 chloride solution, silver chloride alone precipitates. Only 

 after all the chlorine has combined with the silver does the 

 silver chromate separate. 1 This mixes with the precipitated 

 silver chloride and imparts to it an orange color. 



The silver solution is advantageously prepared, so that 

 1 cc. corresponds to 0.01 g. of sodium chloride. This is 

 obtained by dissolving 29.054 g. of pure fused silver nitrate 

 in one liter of water or 7.2635 g. in 250 cc. (For the method 

 of making up the solution see "Determination of Urea," 

 page 181.) 



Determination. To 10 cc. of the urine, in a porcelain dish 

 or in a flask placed on white paper, add 100 cc. of water 

 and then a few drops of a solution of potassium chromate 

 until a distinct yellow color results. Then let the silver solu- 

 tion run in from a burette until the red color, which forms 

 where the silver solution runs in, no longer disappears when 

 the liquid is thoroughly stirred. The first trace of a perma- 

 nent orange color marks the end-point of the reaction. The 

 first titration gives only an approximate result. The deter- 

 mination is to be repeated once more with the same urine. 



XII. DETERMINATION OF THE TOTAL SULPHURIC ACID. 



Heat 100 cc. 2 of filtered, perfectly clear, urine in a beaker 

 on the wire gauze to boiling with 10 cc. of hydrochloric acid. 

 Keep boiling gently for about ten minutes, then remove the 



1 The silver phosphate precipitates after the chromate. 



3 In the case of concentrated urines 50 cc.+ 50 cc. of water are sufficient* 



