NITRATES. 105 



This reduction is brought about by the action of 

 nascent hydrogen, which may be generated by 



(a) An oxidisable metal in presence of an alkali. 



(b) Action of a metal on an acid. 



(c) Electrolysis of water. 



In all cases the main chemical reaction is the same 



one molecule of ammonia being formed from one 

 molecule of nitric acid. 



The ammonia formed is then estimated by distillation 

 with an alkali and reception in a measured quantity of 

 standard acid, the amount of acid neutralised being 

 afterwards determined by titration with standard 

 sodium hydrate exactly as in the Kjeldahl process for 

 total nitrogen. 



In some cases, where the quantity of nitrate present 

 is small, the amount of ammonia formed is estimated 

 by the well-known Nessler's method as used in water 

 analysis. 



Many variations in this process have been devised. 

 Thus, as reducing agents the following have been used : 

 sodium amalgam containing about *5% sodium, alloy of 

 aluminium, copper and zinc in fine powder, the copper- 

 zinc couple, the aluminium-mercury couple, zinc or 

 iron dust, aluminium powder ; these are all applied in 

 alkaline or neutral solution. In acid solution, iron or 

 zinc dust may be employed either in the presence of 

 acetic, or more generally, sulphuric acid. 



In all these cases, after the reduction of the nitrate to 

 ammonia is complete, an operation which goes most 

 quickly, as a rule, in the acid solutions, a large excess 

 of caustic soda or potash is added and the whole dis- 

 tilled, the ammonia being received in sulphuric acid, in 

 which its amount is determined by titration, if it be 

 large, or by Nesslerisation, if it be small. 



(4) The production of a colour with certain organic substances 

 in presence of sulphuric acid and the comparison of the 

 depth of colour produced with that formed by a known 

 amount of pure potassium nitrate under like conditions. 



