NITROGEN. 91 



Tests for nitric acid or nitrates. 

 (Potassium nitrate, KNO 3 , may be used as a nitrate.) 



1. Nitric acid when heated with copper filings, or nitrates when 

 heated with copper filings and sulphuric acid, evolve red fumes of 

 nitrogen tetroxide. (See explanation above.) On the addition of 

 alcohol to the mixture, the odor of nitrous ether is noticed. 



2. The solution of a nitrate, to which a few small pieces of ferrous 

 sulphate have been added, will show a reddish-purple or black color- 

 ation upon pouring a few drops of strong sulphuric acid down the 

 side of the test-tube, so that it may form a layer at the bottom of the 

 tube. The black color is due to the formation of an unstable com- 

 pound of the composition 2FeSO 4 .NO. 



3. Solution of indigo is changed to yellow by nitric acid. Solu- 

 tions of nitrates mixed with dilute sulphuric acid do not bleach 

 indigo in the cold, but do so on heating. 



4. Nitrates deflagrate when heated on charcoal by means of the 

 blowpipe. 



5. When a few drops of a solution of 1 part of brucine in 300 

 parts of 5 per cent, dilute sulphuric acid are added to a very dilute 

 solution of a nitrate, and then some concentrated sulphuric acid is 

 carefully poured down the side of the test-tube, a red color, changing 

 to yellow, is produced at the line of contact. 



6. When a few drops of solution of diphenylamine, NH(C 6 H 5 ) 2 , in 

 concentrated sulphuric acid are added to solution of a nitrate, and 

 then concentrated sulphuric acid is poured down the side of the test- 

 tube, a deep-blue color is formed at the line of contact. 



7. Crystals of pyrogallic acid, C 6 H 3 (OH) 3 , added to solution of a 

 nitrate, and then concentrated sulphuric acid poured down the side 

 of the test-tube, produce a deep-brown color at the line of contact. 



Reactions 5, 6, and 7 show 1 part of nitric acid in one, three, and 

 ten million parts of water respectively, and are used chiefly to detect 

 traces of nitric acid in drinking-water. As sulphuric acid may con- 

 tain nitric acid, the tests should also be made with the sulphuric acid 

 alone in order to prove its purity. 



As an antidote in cases of poisoning by nitric acid a solution of sodium car- 

 bonate, or a mixture of magnesia and water, may be administered with the 

 view of neutralizing the acid. 



QUESTIONS. 111. State the physical and chemical properties of nitrogen. 

 112. Mention the principal constituents of atmospheric air and the quantity in 

 which they are present. 113. By what processes can the four chief constituents 



