Y. THE ANALYSIS AND COMPOSITION OF SOILS 95 



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. 



(3) 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. Several substances give characteristic 

 colours with nitrates and may be used for the determination, the most 

 generally used being carbazole or diphenylimide, (C 6 H 4 ) 2 NH, which 

 in acetic acid solution, gives a deep-green colouration with a nitrate ; 

 phenyl-sulphuric acid, C 6 H 5 HSO 4 , which yields picric acid 

 C 6 H 2 (NO 2 ) g OH with a nitrate and, on the subsequent addition of 

 ammonia, gives an intense yellow colouration ; and brucine, 

 C 23 H,, 6 N. 2 O 4 .4:H 2 O, which with strong sulphuric acid and a nitrate yields 

 a characteristic intense red colouration. 



Determination of ammonia. 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 be- 

 ing received in standard sulphuric acid. Magnesia is employed instead 

 of soda or potash because it has not, like these alkalies, the power of 

 setting free a portion of the nitrogen (as ammonia) from organic nitro- 

 genous compounds. 



Determination of nitrites. Nitrites are, as has been shown in 

 Chapter IV, an intermediate stage in the conversion of organic or 

 ammoniacal nitrogen into nitrates. Their presence in a soil may some- 

 times be detected, but only minute quantities are usually present unless 

 the activity of the nitric organism is not so great as that of the nitrous 



anism. 



Many delicate reactions, mainly colourimetric ones, can be used for 

 their detection and estimation. Metaphenylene diamine, C 6 H 4 (NH 2 ) 2 , 

 gives with nitrous acid an intense yellow colouration, and this coloura- 

 tion, developed by heating, is compared with that obtained by the use 

 of a known quantity of a nitrite. 



A more delicate test, not so readily interfered with by the yellow 

 colour which the soil extract may possess, is the addition of a mixture 

 of sulphanilic acid and naphthylamine in acetic acid. 1 This gives with 

 minute traces of nitrites, when warmed to 70 or 80, a red colouration. 



The reaction is due to the following changes : 



C 6 H 4 NH 2 HS0 3 + UNO, = C,.H 4 {^; ) N 1 + 2H 2 O 



benzene sulphonic acid 3 I 



or sulphanilic acid. ^-diazo benzene 



sulphonic acid. 



C (i H 4 N 2 S0 3 + C IO H 7 NH 3 C 2 HA 



a-naphthylamine (or 

 naphthylammonium) acetate. 



= C 10 H G NH 2 .N:N.C H 4 HS0 3 + HC,H 3 (X 



a-amino naphthalene azo-benzene 



^-sulphonic acid. 

 1 Grriess, Zeitsch. anal. Chem., 18, 597. 



