URINE 287 



of oxalate. Consider the result in connection with your ob- 

 servations in the preliminay experiments under section (1). 



3. Total Nitrogen. (Kjeldahl Method). 



This method is of utmost importance and is widely used for 

 estimating total nitrogen. The various nitrogen compounds 

 are broken down by heating with concentrated sulphuric acid, 

 the nitrogen being converted into ammonia, and the carbon into 

 carbon dioxide. The ammonia is retained in the solution as 

 (NH 4 ) 2 S0 4 . It then is liberated by the addition of sodium 

 hydrate and distilled into a known volume of N/10 hydrochloric 

 or sulphuric acid. The excess of N/10 acid is then determined 

 by titration and the amount of ammonia calculated. 



In analyzing liquids it is customary to use 5 or 10 c.c., ac- 

 cording to the nitrogen content. If the material is a solid, 1 

 gram accurately weighed is the usual amount. 



With a pipette measure 5 c.c. urine into a 500 c.c. Kjeldahl 

 flask. Add 8-10 g. potassium sulphate which raises the boiling 

 point, 15 c.c. of concentrated H 2 S0 4 and 2 c.c. of 5% copper 

 sulphate which acts as a catalyser. 



Heat the flask in an inclined position over a small flame until 

 the contents become clear and pale green. There must be no 

 suggestion of yellow and no black specks of unoxidized carbon 

 anywhere in the liquid or on the inner surface of the flask. If 

 black specks are present, remove the flask from the flame and 

 by careful shaking, rinse them down. Do not attempt to rinse 

 them down by adding water. 



Note: Injurious fumes are given off during the heating 

 in the Kjeldahl analysis, so that it should never be carried 

 out in the open laboratory, but in a well drawing hood, or 

 other arrangement for carrying off the fumes. 



When digestion is complete, allow the liquid to cool and 

 about half fill the flask with distilled water. The nitrogen now 

 is in solution in the form of ammonium sulphate. It must be 

 set free as ammonia and driven over by distillation into a 

 known volume of standard acid. 



