1800.] CJtange between Niti*ic Acid and certain .!/* /'//*. 459 



nitric acid of about 30 per cent, concentration (the acid commonly 

 employed for the preparation of nitric oxide gas) and heated to a 

 temperature of 30 C., provided that nitrous acid is neither present 

 initially nor formed subsequently. To prevent this, it is necessary in 

 the cases of copper and bismuth to add a small quantity of some 

 oxidising substance, such as hydrogen peroxide or potassium chlorate, 

 or, as less efficacious, potassium permanganate, or to pass a current of 

 air or, lastly, such a substance as urea, which destroys the nitrous 

 acid by its interaction. 



II. If the conditions are such that these metals dissolve, then the 

 amount of metal dissolved and the amount of nitrous acid present are 

 concomitant variables, provided that the nitric acid is in considerable 

 excess. Change of conditions, such as concentration of acid and 

 variation of temperature, which increase the former increase also the 

 latter. 



III. If the conditions are such that these metals dissolve, it would 

 appear that the metallic nitrite is at first formed, together with nitric 

 oxide; the former is decomposed by the excess of nitric acid to 

 liberate nitrous acid, while the latter reduces the nitric acid to form 

 a further quantity of nitrous acid. 



Eventually the net result is the product of two reverse chemical 

 changes represented by the equations 



(i.) 2 



(ii.) 3HN0 3 = 



The nitrous acid is thus destroyed as fast as it is generated. 

 I V, If the conditions are such that metals dissolve in nitric acid, 

 then nitrous acid is invariably the initial product of reduction. 



V. The metals copper, mercury, and bismuth dissolve very readily 

 in a 1 per cent, solution of nitrous acid ; under these conditions nitric 

 acid present in slight excess interferes with, rather than promotes, the 

 chemical change. This result is probably due to the greater stability 

 of nitrous acid in the presence of nitric acid. 



VI. Hydrogen gas reduces nitric to nitrous acid in presence of 

 cupric or lead nitrate ; it also converts mercuric into mercurous 

 nitrate, but does not produce any change in solutions of bismuth and 

 zinc nitrates dissolved in nitric acid. 



