426 



A MANUAL OF PHYSIOLOGY 



distillation for two or three minutes longer. The ammonia is now 

 all united with the standard acid, a certain amount of which is left 

 over. By determining this amount we arrive at the quantity com- 

 bined with ammonia, and therefore at the quantity of ammonia. 

 Fill a burette with a decinormal solution of potassium or sodium 

 hydrate.* Add a little methyl orange solution to the standard 

 sulphuric acid, to serve as indicator. Then run in the potassium 

 or sodium hydrate till the pink tinge gives place to a permanent but 

 just recognisable yellow. Let x be the number of c.c. run in. 

 Since i c.c. of any decinormal solution is equivalent to i c.c. of any 

 other, vT represents also the number of c.c. of the standard sulphuric 



FIG. 132. ARRANGEMENT FOR DISTILLATION IN ESTIMATION OF TOTAL 



NITROGEN. 



acid left uncombined with ammonia ; and 50-^, the quantity com- 

 bined with ammonia. Then, i c.c. of decinormal sodium or potassium 

 hydrate being equivalent to i c.c. of decinormal ammonium hydrate, 

 and i c.c. of decinormal ammonium hydrate containing "0014 

 gramme nitrogen, we get (50 - x) x -0014 as the quantity of nitrogen 

 in 5 c.c. of urine. 



9. Uric Acid (i) Preparation. Uric acid can be prepared in a 

 pure form from serpents' excrement, by dissolving it in dilute sodium 

 hydrate, and filtering. The filtrate contains sodium urate, which is 



* A normal solution of a substance contains in a litre a number of 

 grammes of the substance equal to the number which expresses its 

 equivalent weight a decinormal solution one-tenth of this amount, a 

 centinormal one-hundredth, etc. Thus, a normal solution of potassium 

 hydrate contains 56 grammes of KOH, and a decinormal solution 5 '6 

 grammes in 1000 c.c. 



