106 THE ANALYSIS OF SOILS. V. 



Many substances have been used for the purpose of 

 yielding the colour thus, carbazole or diphenylimide, 



( p r ' 4 JNH, phenyl-sulphuric acid, CH 5 HS0 4f bru- 



( O(;-tl-4 ) 



cine, C. 28 H. r) N,0 4 .4H 2 0. 



In the case of carbazole, an acetic acid solution of the 

 substance is added to pure sulphuric acid. The mix- 

 ture gives, when added to a liquid containing a nitrate 

 even in minute quantities, a deep green colouration, the 

 intensity of which is compared with that given with a 

 solution of potassium nitrate of known strength after 

 the manner of Nessler's ammonia process. This re- 

 action is only reliable in the absence of chlorides and 

 iron compounds. These have, therefore, to be removed 

 from the soil extract. 



Phenyl sulphuric acid is converted into picric acid by 

 nitrates and the ammonium picrate formed on the sub- 

 sequent addition of ammonia is possessed of a strong 

 yellow colour. 



The reaction is 



Phenyl sulphuric acid. Picric acid. 



and on the addition of ammonia 

 C 6 H a (NO a ) 8 OH + NH 4 OH - C (; H. 2 (NO 2 ) 8 ONH 4 + H. 2 0. 

 Brucine gives with a nitrate in the presence of strong 

 sulphuric acid an intense red colouration. 

 In all cases these reactions are applicable to the determi- 

 nation of minute quantities of nitrates, and it is obvious that 

 only the purest reagents must be employed. 



If a determination of ammonium salts in a soil be desired, 

 distillation of a weighed quantity of the soil with pure water 

 and magnesia is performed, the ammonia evolved being re- 

 ceived in standard sulphuric acid. Magnesia is employed 

 instead of soda or potash because it has not, like these alkalis, 

 the power of setting free a portion of the nitrogen (as am- 

 monia) from organic nitrogenous compounds. 



Determination of nitrites. Nitrites are, as has been shown 

 in Chap. IV., an intermediate stage in the conversion of organic 

 or ammoniacal nitrogen into nitrates. Their presence in a 



