TOTAL NITROGEN . 33 



Long's coefficient was determined for urine whose specific 

 gravity was taken at 25 C. and is probably more accurate, for 

 conditions obtaining in America, than the older coefficient of 

 Haeser, 2.33. 



TOTAL NITROGEN 



Kjeldahl Method 



Principle. The principle of this method is the conversion of 

 the various nitrogenous bodies of the urine into ammonium 

 sulphate by boiling with concentrated sulphuric acid, the sub- 

 sequent decomposition of the ammonium sulphate by means of 

 a fixed alkali (NaOH) and the collection of the liberated ammonia 

 in an acid of known strength. Finally, this partly neutralized 

 acid solution is titrated with an alkali of known strength and the 

 nitrogen content of the urine under examination computed. 



Procedure. Place 5 c.c. of urine in a 700 c.c. Kjeldahl flask, 

 add 20 c.c. of concentrated sulphuric acid and about 0.2 gm. of 

 copper sulphate, or 10 gms. of potassium sulphate and boil the 

 mixture for some time after it is colorless (about one-half hour). 



Allow the flask to cool and dilute the contents with about 

 200 c.c. of ammonia-free water. Add a little more of a concen- 

 trated solution of NaOH than is necessary to neutralize the sul- 

 phuric acid and introduce into the flask a little coarse pumice 

 stone or a few pieces of granulated zinc, to prevent bumping, 

 and a small piece of paraffin to lessen the tendency to froth. By 

 means of a safety-tube connect the flask with a condenser so 

 arranged that the delivery-tube passes into a vessel containing 

 a known volume (the volume used depending upon the nitrogen 

 content of the urine) of N/10 sulphuric acid, using care that the 

 end of the delivery-tube reaches beneath the surface of the fluid. 

 Mix the contents of the distillation flask very thoroughly by shak- 

 ing and distill the mixture until its volume has diminished about 

 one-half. Titrate the partly neutralized N/10 sulphuric acid 

 solution by means of N/10 sodium hydroxide, or ammonium 

 hydroxide, using Congo red as indicator, and calculate the content 

 of nitrogen of the urine examined. 



Calculation. Subtract the number of cubic centimeters of 

 N/10 sodium hydroxide used in the titration from the number 



