PRACTICAL EXERCISES 483 



continue the 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 combined with ammonia, and therefore at the quantity of 

 ammonia. Fill a burette with a decinormal solution of potassium 

 or sodium hydroxide. Add a little methyl-orange solution to the 

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

 potassium or sodium hydroxide till the pink tinge gives place toja 

 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, x represents also the number of c.c. of the standard 

 sulphuric acid left uncombined with ammonia ; and 50 - x, the 

 quantity combined with ammonia. Then, i c.c. of decinormal 

 sodium or potassium hydroxide being equivalent to i c.c. of deci- 



FIG. 180. ARRANGEMENT FOR DISTILLATION IN ESTIMATION OF TOTAL 

 NITROGEN. 



normal ammonium'! hydroxide,^and i } c.c. of decinormal ammonium 

 hydroxide containing o - ooi4'gramme nitrogen, we get (50 x) x 0^0014 

 as the quantity of nitrogen in 5 c.c. of urine. 



Instead of mercury, potassium sulphate and copper sulphate may 

 be added to the sulphuric acid in order to aid the decomposition in 

 the first stage of the estimation. About 3 grammes of potassium 

 sulphate and i gramme of copper sulphate are added to 5 c.c. of 

 urine, and then 5 c.c. of sulphuric acid. The liquid is gently boiled 

 for an hour, or until it is quite clear. The neutralization and dis- 

 tillation are conducted as before, the proper quantity of sodium 

 hydroxide being determined in advance. No potassium sulphide is 

 added, but a small quantity of talc may be put in to prevent bump- 

 ing. Instead of methyl orange. ' alizarin red,' which is bright 

 red in thel presence of the slightest trace of alkali, may be used. 



312 



