432 ANALYTICAL CHEMISTRY. 



by digesting the water with a little sodium carbonate and sodium hydroxide 

 before Nesslerizing. 



Nitric acid. While there are methods by which nitric acid can be deter- 

 mined more accurately, it often suffices to make the tests with brucine, 

 diphenylamine, and pyrogallic acid, as described under the analytical reactions 

 of nitric acid on page 177. 



Nitrous acid. A solution made by dissolving 1 gm. of metaphenylene- 

 diamine in 200 c.c. water, containing 5 gm. H 2 SO 4 , is used for the determina- 

 tion of nitrites in the same manner as Nessler's solution is used for ammonia. 

 The required standard nitrite solution is prepared by dissolving 0.406 gm. silver 

 nitrite and 0.225 gm. potassium chloride in hot water, mixing and, after cool- 

 ing, filling up to 1 liter. After filtering off the precipitated silver chloride, 

 lOo'c.c. of the nitrate are diluted to 1000 c.c. This solution contains nitrous 

 acid equivalent to 10 mg. of N 2 O 3 per liter. 



Metaphenylene-diamine solution, prepared as above, gives with nitrites a 

 yellow color, the intensity of which serves for the quantitative estimation of the 

 nitrites present. The test is made by using definite dilutions of the above 

 standard nitrite solution in the four test-cylinders, adding 1 c.c. of the meta- 

 phenylene-diamine to each 50 c.c., and comparing the colors produced with that 

 obtained in the water specimen, treated in like manner. Immersion of the 

 cylinders in warm water accelerates the reaction. 



Sulphates. While there are volumetric methods for the determination of 

 sulphates, this can be conveniently made by the gravimetric method i. e. t by 

 precipitating the sulphate with barium chloride and weighing the precipitated 

 BaS0 4 . From 100 to 250 c.c. of water should be used. 



Chlorine. If the water under examination has had an opportunity to 

 become charged with sodium chloride from its proximity to the sea coast, to 

 salt lakes, or by flowing through strata containing salt deposits, then a con- 

 siderable quantity of chlorides may be present and yet the water may be used 

 without detriment to health. But in other cases the chlorides are derived 

 from cesspools, sewage, etc., and their presence is then indicative of dangerous 

 pollution. 



The determination of chlorine is made by titrating 100 c.c. of water with 

 ^ AgN0 3 , using potassium ~,hromate as an indicator. For water containing 

 much organic matter the gravimetric method should, be used. 



Phosphates. The ammonium molybdate test (see analytical reactions of 

 phosphoric acid, page 228) should give no indication of phosphoric acid, as the 

 presence of soluble phosphates in water is almost positive proof that pollution 

 with urine has taken place. 



QUESTIONS. Explain the principles which are made use of in gravimetric 

 and volumetric determinations. Give an outline of the operations to be per- 

 formed in the gravimetric determination of copper in cupric sulphate. What 

 are normal and deci-normal solutions, and how are they made? What is the 

 use of indicators in volumetric analysis ? Mention some indicators and explain 



