99] Nitrogen in Presence of Nitrates 67 



Strong caustic soda and standard acid and alkali as before 

 (paragraphs 87 and 91). 



Zinc-dust. 



99. The Operation. — Weigh out about a gram of the 

 mixture of starch and ammonium nitrate. Transfer to an 

 8-oz. flask and moisten with the smallest possible quantity of 

 water. Take especial care that no dry particles are left in the 

 flask ; for if such particles should float on the surface of the 

 acid when it is poured into the flask, HN0 3 may be given off 

 and escape. Pour into the moistened substance 30 c.c. of 

 sulphuric acid in which 2 grams of salicylic acid has been dis- 

 solved. Heat on a stand (fig. 28) or sand bath until all froth- 

 ing ceases. Whilst this preliminary heating is going on, weigh 

 out roughly 2 grams of zinc-dust. As soon as the frothing 

 has ceased add this zinc-dust, a little at a time ; then add 

 about 10 grams of K 2 S0 4 and heat until the liquid becomes 

 colourless. If, in an hour, the liquid is still black or contains 

 carbonaceous matter, the clearing may be greatly assisted by 

 the addition of a drop of mercury. 



When the liquid is quite clear, all the nitrogen contained 

 in the original substance will be in the form of ammonium 

 sulphate. If the operation has been managed satisfactorily 

 without the aid of mercury, the ammonia may be driven off 

 by 150 c.c. of strong caustic soda, as described in paragraphs 

 93-95. Should it have been found necessary to use mercury, 

 it must be removed from solution by adding 25 c.c. of the 

 sodium sulphide solution together with the caustic soda. The 

 reason for this is that mercury-ammonium salts are apt to be 

 formed, which are not completely decomposed by the strong 

 alkali, and hence some of the ammonia does not find its way 

 into the standard acid. 



F2 



