104 THE ANALYSIS OF SOILS. V. 



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 c.c. have percolated 

 it may be assumed that all the nitrates are removed 

 (Warington). 



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 

 measurement of the volume of this gas, 

 This can be effected either by 



ScJiloesing's method, in w r hich the nitrate is treated 

 with ferrous chloride and strong hydrochloric acid 



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 + 2KN0 3 - 3Hg a SO 4 + K 2 SO 4 

 + 4H 2 + 2NO. 



This latter method is vitiated if any appreciable quan- 

 tity of organic matter be present. The former method 

 gives good results, even in the presence of organic 

 matter. The Crum-Frankland method can be con- 

 veniently carried out in Lunge's nitrometer, for a 

 description of which any treatise on quantitative analy- 

 sis may be consulted. 



(2) The dccolorisation of a solution of indigo by the nitrate in 



the presence of sulphuric acid. 



The amount of nitric acid is then estimated by com- 

 parison with the quantity of a solution of potassium 

 nitrate of known strength required to decolorise an 

 equal volume of the same indigo solution, under similar 

 conditions of temperature and concentration. 



This process is also subject to interference by the 

 presence of organic matter and is now rarely used for 

 soils. 



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



determination of the amount of ammonia so formed. 



