NITRITES. 107 



soil may sometimes 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 organism. 



Many delicate reactions, mainly colorimetric ones, can be 

 used for their detection and estimation. Metaphenylene dia- 

 mine, C (i H 4 (NH 2 ) 2 , gives with nitrous acid an intense yellow 

 colouration, and this colouration, 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 addi- 

 tion of a mixture of sulphanilic acid and naphthylamine in 

 acetic acid." 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 f) H 4 NH 2 HS0 3 + HN0 2 = CH 4 |^} + 2H 2 



*>-amido benzene sulphonic acid , * ' 



or sulphanilic acid. Hj 1 ? Benzene 



sulphonic acid. 



C H 4 N a S0 8 + C 10 H 7 NHAH 8 2 



a-naphthylamine or 

 na-phthylammonium acetate. 



= C 10 H (; NH 2 .N:N.C 6 H 4 HSO S + HC 8 H 8 O a 



a-amido naphthalene azo-benzene 

 jo-sulphonic acid. 



This complex compound has intense tinctorial power and is 

 the cause of the red colouration. 



Another reaction for nitrites of considerable delicacy, though 

 liable to interference by the presence of organic matter, is the 

 liberation of iodine from hydriodic acid and its detection by 

 the formation of the blue colour with starch. 



Griess, Zeits. fitr analyt. Chem. 18, 597. 



