PHYSIOLOGICAL CHEMISTRY 



413 



The chemical reaction involved in this stage of the method is the 

 liberation of ammonia from the ammonium hydrogen sulphate (pro- 

 duced in the incineration stage) by the excess of alkali added to the 

 contents of the distilling flask. This ammonia is completely carried 

 over into the sulphuric acid of the receiver during the distillation, and 

 thus quantitatively neutralises some of the acid. 



Titration (3rd stage). Cool the receiver under the tap. Run in =^\ 

 caustic soda carefully from a burette, until its contents are neutral, 



FIG. 239. Apparatus for determination of total nitrogen. 



as shown by the change in colour of the indicator employed. Subtract 

 the volume in c.c. of -^ soda required for neutralisation from the 



volume of -^ acid originally added to the receiver. The remainder 



represents the volume in c.c. of y^ acid neutralised by ammonia during 



the distillation. Let this remainder be n c.c. Then, as the ammonia 

 distilled quantitatively represents the total nitrogen present in the 

 5 c.c. of urine taken, this 5 c.c. of urine contains nx 0*0014 grm. 



wxO-OOUxlOO 

 nitrogen, and, therefore 100 c.c. of the urine contain - ^ 



grm. nitrogen. 



To take an example, suppose that 50 c.c. ^ acid were placed in the 



receiver, and that after the distillation 19 '4 c.c. ^ alkali were required 



for neutralisation. Then 5 c.c. urine contain 



(50 - 19-4) x 0-0014 grm. = 0-0428 grm. nitrogen. 

 Therefore 100 c.c. urine contain 0-856 grm. nitrogen. 



