METAPHENYLENEDIAMIN METHOD 467 



and chlorates. The prevailing colors developed with nitrates are 

 reddish yellow, green and dirty violet, and with chlorates, red- 

 brown, green and gray. The reactions, therefore, as usually pro- 

 duced are not always reliable, and Alvarez has proposed modi- 

 fications of the process as follows : 86 



The reagent is prepared by dissolving o.i gram of diphenyl- 

 amin and the same quantity of re-sublimated resorcin and add- 

 ing five or six drops of the solution to a fragment of the salt 

 to be tested (nitrites, nitrates or chlorates of the alkalies), placed 

 in a flat-bottomed porcelain capsule. 



The following phenomena are observed: With nitrates a very 

 permanent yellowish green tint is obtained, and after spreading 

 out over the surface of the dish the edges of the spot become 

 blue. By the addition of alcohol an orange color is obtained. 

 With nitrites a deep blue violet color is produced and by moving 

 the liquid so as to wet the whole interior of the dish, the edges 

 become red. On adding alcohol a red color is formed. With 

 chlorates the results are not satisfactory. 



ESTIMATION OF NITROUS ACID BY COLORIMETRIC 

 COMPARISON 



401. Application of the Method. The most minute traces of 

 nitrous acid may be detected by colorimetric methods, and the 

 determination of the quantity present may be approximated with 

 great exactness by comparison with a solution of a nitrite of 

 known strength. Especially in following the progress of nitri- 

 fication is this method, in some of its forms, of essential import- 

 ance. In delicacy and celerity it has the same advantages as the 

 colorimetric methods for the determination of nitric acid, and 

 the same skill must be exhibited and the same precautions ob- 

 served as in the case of nitric acid in order to avoid errors. 



402. Metaphenylenediamin Method. This process depends 

 upon the development of a yellow color in water containing 

 nitrous acid on the addition of a reagent containing metaphenyl- 

 enediamin; (m C G H 4 (NH 2 ) 2 ). This variety of the phenylene- 

 diamins is readily obtained from common dinitrobenzene. It melt? 



** Chemical News, 1905, 91 : 155. 



