AGRICULTURAL ANALYSIS 



at 63 9 and boils at 287. In order to preserve the reagent in 

 shape for use it should be prepared in the following manner: 



Dissolve two grams of the chlorid in 10 cubic centimeters of 

 ammonia, and place the solution in a glass-stoppered flask. To 

 this solution are added five grams of powdered animal-black, and 

 the whole vigorously shaken. After allowing to settle, the shak- 

 ing is repeated at intervals of an hour, three or four times, and 

 the flask then allowed to remain at rest for 24 hours. 



The supernatant liquid is generally sufficiently decolorized by 

 this treatment. If not, the shaking and subsidence must be re- 

 peated until a completely colorless liquid is obtained. The solu- 

 tion can be kept indefinitely in contact with the animal-black. 

 Aqueous and alcoholic solutions of the salt can not be kept. 



The test is made by mixing five drops of the reagent with five 

 cubic centimeters of sulfuric acid. The mixture must be color- 

 less. To the- mixture add 100 cubic centimeters of the water or 

 solution to be tested, and heat on the water bath for five minutes. 

 A yellow coloration indicates the presence of nitrous acid. 



The metaphenylenediamin test is fairly satisfactory in perfectly 

 colorless waters and aqueous extracts. Many waters and soil 

 and fertilizer extracts, however, have a yellowish tint, and this 

 interferes in a marked way with a proper judgment of the yellow 

 triaminazobenzol developed in the application of the above test. 



The decoloration of such waters by means of sodium carbonate 

 or aluminum hydroxid is a matter of some difficulty, and not 

 wholly without action on the nitrites which may be present. The 

 method, therefore, is inferior to the one next described. 



403. Sulfanilic Acid and Naphthylamin Test for Nitrous Acid. 

 A very delicate test for the presence of nitrous acid, first de- 

 scribed by Griess, is the coloration produced thereby in an acid 

 solution of sulfanilic acid and naphthylamin. 87 



87 Berichte der deutschen chemischen Gesellschaft, 1879, 12 : 426. 

 Zeitschrift fur analytische Chemie, 1879, 1 8 : 597. 

 Zeitschrift fur angewandte Chemie, 1889, 2 : 666. 

 Bulletin de la socie*te* chimique de Paris, 1889, [3], 2 : 347. 

 This work, 1 : 533. 



