314 SMITH'S INTERMEDIATE CHEMISTRY 



r>f riving nf 



^vapor. The oxides of nitrogen can be recovered by absorption 

 in water, yielding dilute nitric acid. This can be concentrated 

 further if desired, or neutralized with a base for the production 

 of nitrates for use as explosives (ammonium nitrate) or fertilizers 

 (calcium nitrate). 



Platinum gauze is almost universally employed as a catalyst. 

 A mixture of ammonia and oxygen-enriched air passes through 

 one or more layers of the gauze, which is heated electrically to 

 650-700 to start the reaction and which is maintained at that 

 temperature subsequently by the heat of combustion of the 

 ammonia. The reaction that takes place may be represented by 

 the equation: 



4NH 3 + 50 2 - 4NO + 6H 2 O. 



The excess oxygen present converts the NO into NO 2 as the issu- 

 ing gases cool. Unless the reaction is very nicely regulated, 

 however, the yield of oxides of nitrogen is diminished either by 

 incomplete combustion of ammonia, or by the dissociation of 

 NO into nitrogen and oxygen (see p. 312). The time of contact 

 with the catalyst must not be too long, or this latter effect will be 

 appreciable. With proper precautions, a conversion efficiency of 

 90-95 per cent is obtained. Poisoning of the catalyst must be 

 guarded against by careful purification of the gases, especially 

 from non-volatile impurities such as dust particles, which choke 

 the surface of the gauze and render it inoperative. 



Nitrous Acid HNO2 and Nitrous Anhydride N 2 O3. 



When an acid, such as sulphuric acid, is added to a solution of 

 a nitrite, like potassium nitrite (p. 311), nitrous acid HN0 2 is 

 formed : 



2KN0 2 + H 2 S0 4 -* K 2 S0 4 + 2HNO 2 . 



Nitrous acid, however, like sulphurous acid, is unstable and 



nitrous anhydride N 2 Os is at once liberated and escapes as a gas: 



2HNO 2 ->H 2 O-f 



