NITROGEN. 77 



The usual method for obtaining nitric acid is the decompo- 

 sition of sodium nitrate by sulphuric acid : 



H 2 S0 4 = HNO 3 + NaHSO 4 ; 



Bisulphate 

 of sodium. 



Or 



2NaN0 3 -f H 2 S0 4 = 2HNO 3 + Na 2 SO 4 . 



Neutral sul- 

 phate of sodium. 



The operation is accomplished by mixing the two substances 

 in a retort and heating, when nitric acid distils over and is col- 

 lected in a suitable receiver, whilst sulphate of sodium remains 

 in the retort. 



The acid thus obtained is an almost colorless, fuming, corro- 

 sive liquid, of a peculiar, somewhat suffocating odor, and a 

 strongly acid reaction. 



Common nitric acid of a specific gravity 1.42, is composed of 

 69.4 per cent, of HN0 3 and 30.6 per cent, of water. The di- 

 luted nitric acid of the U. S. P. is made by mixing one part of 

 the common acid with six parts of water. 



Nitric acid is completely volatilized by heat : it stains animal 

 matter distinctly yellow ; it is a monobasic acid forming salts 

 called nitrates. These salts are all soluble in water, for which 

 reason nitric acid cannot be precipitated by any reagent. 

 Mtric acid is a strong oxidizing agent ; this means it is capable 

 of giving off part of its oxygen to substances having affinity 

 for it. 



Tests for nitric acid or nitrates. 



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

 when heated with copper filings and sulphuric acid, evolve red 

 fumes of nitrogen tetroxide. 



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

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

 )lack coloration 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. 



3. Solution of indigo is decolorized by nitric acid. 



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

 of carbonate of sodium, or a mixture of magnesia and water, 

 may be administered with the view of neutralizing the acid. 



