94 DETERMINATION OF NITRATES 



From the air-dried soil the nitrates are extracted by washing with 

 water. Many methods may be used : 



(1) 1000 grammes are treated with 2 litres of pure water and al- 

 lowed to stand for forty-eight hours, with occasional shaking. 1 litre of 

 the liquid is then filtered, mixed with a little pure sodium carbonate, and 

 evaporated to small volume on the water-bath, any precipitate which 

 may form being removed by filtration. 



(2) A cylindrical funnel is made by removing the bottom from a 

 " Winchester quart " bottle, placing a disc of copper gauze on the 

 shoulder, and covering this with two filter papers. The bottle is then 

 connected by means of a cork and tube to a filtering flask and from 

 200 to 500 grammes of dried soil are placed in it, resting on the paper- 

 covered wire gauze. Water is then poured in until the whole of the 

 soil is moistened, small quantities of water are added from time to 

 time, and the filter pump started gently. When 100 cc. have per- 

 colated it may be assumed that all the nitrates are removed (War- 

 ington). 



In the extract obtained, the amount of nitric nitrogen may be 

 determined by one of the following methods : 



(1) The reduction of the nitric acid to nitric oxide and the measure- 

 ment of the volume of this gas. 



This can be effected either by 



Schloesing's method, in which the nitrate is treated with ferrous 

 chloride and strong hydrochloric acid 



6FeCl 2 + 2KNO 3 + 8HC1 = 6FeCl 3 + 2KC1 + 4H,O + 2NO. 



or Crum-Frankland's method, in which by the action of metallic 

 mercury and sulphuric acid the nitrate yields its nitrogen as nitric 

 oxide 



6Hg + 4H 2 S0 4 + 2KNO 3 = 3Hg 2 S0 4 + K 2 SO 4 + 4H,0 + 2NO. 



This latter method is vitiated if any appreciable quantity of organic 

 matter be present. The former method gives good results, even in the 

 presence of organic matter. The Crum-Frankland method can be 

 ^conveniently carried out in Lunge's nitrometer, for a description of 

 which any treatise on quantitative analysis may be consulted. 



(2) The reduction of the nitrates to ammonia and the subsequent 

 'determination of the amount of ammonia so formed. 



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 



HNO 3 + 4H 2 = NH 3 + 3H 2 O 



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 



