184 QUANTITATIVE ANALYSIS. 



indeterminate error due to the presence of sodium chloride 

 in the urine. This reacts with the mercuric nitrate to form 

 mercuric chloride and sodium nitrate. It is customary, in 

 order to diminish this error, to subtract a certain quantity 

 from the amount of mercury solution used, 1 cc. for dilute, 

 1.5 cc. for concentrated urines (so-called correction for salt), 

 but this correction is entirely arbitrary. 



The above-described simple method, which is sufficient 

 for the physician, has been very much improved by Pfliiger; 

 since, however, the description of Pfluger's method would 

 require too much space, the reader is referred to the original 

 work of Pfliiger or to the larger text-books. 



2. DIRECT DETERMINATION OF NITROGEN IN THE URINE 



ACCORDING TO THE KjELDAHL METHOD. 



This method consists in the conversion of all the nitroge- 

 nous substances in the urine into ammonium sulphate, car- 

 bon dioxide, and water by heating them with concentrated 

 sulphuric acid. Then add an excess of sodium hydroxide, 

 distil off the ammonia into a receiver containing a definite 

 quantity of an acid of known strength (standard acid), and de- 

 termine by means of a standard' solution of ammonia that part 

 of the acid which has not been neutralized by the ammonia. 



It is best to use a fifth-normal solution of hydrochloric 

 acid and a tenth-normal solution of ammonia. These are 

 prepared as follows : 



Preparation of the Fifth-normal Solution of Hydrochloric Acid. 



Dilute 19 cc. of pure concentrated hydrochloric acid, 

 specific gravity 1.19, to 1100 cc. and mix thoroughly. To 

 25 cc. of this dilute acid add 2 cc. of strong nitric acid, specific 

 gravity 1.2, and then a slight excess of a dilute solution of 

 silver nitrate. An excess of silver nitrate may be shown to 

 be present by allowing the precipitated silver chloride to 



