TEST FOR NITROUS ACID 469 



Sulfuric or acetic acid may be used as the acidifying agent, 

 preferably the latter. The solutions are prepared as follows : 



(1) Dissolve one-half gram of sulfanilic acid in 150 cubic cen- 

 timeters of dilute acetic acid. 



(2) Boil o.i gram of naphthylamin with 20 cubic centimeter? 

 of water, decant the colorless solution from the residue and acid- 

 ify it with 150 cubic centimeters of dilute acetic acid. 



The two solutions may at once be mixed and preserved in a 

 well-'stoppered flask. The action of light on the mixture is not 

 hurtful, but air- should be carefully excluded because of the 

 traces of nitrous acid which it may contain. Whenever the mixed 

 solutions show a red tint it is an indication that they have ab- 

 sorbed some nitrous acid. The red color may be discharged and 

 the solution again fitted for use by the introduction of a little 

 zinc dust, and shaking. 



The water, or aqueous solution of a soil or fertilizer, to be 

 tested for nitrites, is treated in portions of about 20 cubic centi- 

 meters with a few cubic centimeters of the mixed reagent and 

 warmed to 7O-8o. If nitrous acid, in the proportion of one 

 part to one million be present, the red color will appear in a few 

 minutes. If the content of nitrous acid be greater, e. g., one part 

 in one thousand, only a yellow color will be produced, unless a 

 greater quantity of the reagent be used. 



Leffmann recommends the following method of conducting 

 the determinations : 88 



Solutions required: i-^-amidobensenesulfonic acid solution 

 (sulfanilic acid). Dissolve 0.5 gram in 150 cubic centimeters 

 of diluted acetic acid, sp. gr. 1.04. 



a-amidonaphthalene acetate solution. Boil o.i gram of solid 

 a-amidonaphthalene (a-naphthylamin) in 20 cubic centimeters 

 of water, filter the solution through a plug of washed absorbent 

 cotton, and mix the filtrate with 180 cubic centimeters of dilut- 

 ed acetic acid. All water used must be free from nitrites, and 

 all vessels must be rinsed out with such water before tests are 

 applied, since appreciable quantities of nitrites may be taken 

 up from the air. 



M Examination of Water for Sanitary and Technical Purposes, 5th 

 edition, 1903 : 54. 



