TEST FOR NITROUS ACID 471 



ment. At the end of this time the two solutions are compared, 

 the colors equalized by diluting the darker, 'and the calculation 

 made as explained under the estimation of nitrates. 



The reactions consist in the conversion of the sulfanilic acid 

 into diazobenzenesulfonic anhydrid, by the nitrite present; this 

 compound is then in turn converted by the amidonaphthalene 

 into azo-a-amidonaphthalene-i-4-benzenesulfonic acid. The last 

 named body gives the color to the liquid. 



The method pursued by Tanner, in the preparation of the 

 reagents, is as follows : 



Sulfanilic acid is prepared by mixing 30 grams of anilin 

 slowly, with 60 grams of fuming sulfuric acid, in a porcelain 

 dish. The brown, sirupy liquid formed is carefully heated until 

 quite dark in color, and until the evolution of sulfurous fumes is 

 noticed. After cooling, the thick, semi-fluid mass is poured into 

 half a liter of cold water and allowed to stand for some hours. 

 The liquid portion is decanted from the nearly black undis- 

 solved crystalline mass. To the residue half a liter of hot water 

 is added and allowed to stand until cold, and the liquid again 

 decanted. The undissolved portion is treated with one liter of 

 hot water and filtered. The filtrate is treated with animal char- 

 coal to decolorize it, and allowed to stand for 24 hours and again 

 filtered, the filtrate diluted to 1500 cubic centimeters and used as 

 required. This solution tends to turn pink on keeping, and thus 

 its color interferes with the delicacy of the test, and a small 

 amount of animal-char is kept in a small bottle containing the 

 portion for immediate use, and this bottle is filled, from time to 

 time, from the larger one. 



The solution of naphthylamin hydrochlorate is made with one 

 gram of the salt dissolved in 100 cubic centimeters of water. 

 The solution is to be occasionally filtered, and not more than 

 100 cubic centimeters should be prepared at a time. 



The analytical operations are carried on as follows : 



A standard solution of pure potassium nitrite, made from tht 

 silver salt in distilled water perfectly free from nitrites, is placed 

 in a color-glass, similar to those used in the nessler reaction, 



