47 2 AGRICULTURAL ANALYSIS 



together with a second glass containing the water to be tested. 

 These glasses should be marked to hold 100 cubic centimeters at 

 the same depth. To each of the tubes a few drops of pure hydro- 

 chloric acid are added and two cubic centimeters of the sulfanilic 

 solution. Afterwards, to each tube are added two cubic centi- 

 meters of the solution of naphthylamin hydrochlorate, and it 

 is allowed to stand for 20 minutes, at the end of which time 

 the color is fully developed. Each tube is covered by a piece of 

 glass in order to prevent access of air. It is unnecessary to add 

 that the standard solutions of nitrite of different strength should 

 be employed until the one is found which resembles, as nearly 

 as possible, the color developed in the sample of water or extract 

 under examination. The application of this test in ascertaining 

 the progress of nitrification is described in Volume I, page 532. 



404. Method of Mason. Mason prefers the old method of 

 using as reagents azobenzolnaphthlamin sulfonic acid. 89 He 

 prepares his reagents as fallows : 



Sulfanilic acid. Dissolve one gram of the salt in 100 cubic 

 centimeters of hot water. The solution keeps well. 



Naphthylamin hydrochlorid. Boil one-half gram of the salt 

 with 100 cubic centimeters of water for ten minutes, keeping the 

 volume constant. Place in a glass stoppered bottle. The solu- 

 tion tends to grow slightly pink on standing, but not sufficient- 

 ly so as to interfere with its use. 



Standard solution of sodium nitrite. Pure sodium nitrite may 

 be used but the silver nitrite as prepared above is preferred. 



Determination. To determine nitrites place 100 cubic centi- 

 meters of the water to be examined (decolorized with aluminum 

 hydrate if necessary) in a nessler tube. Acidify with one drop 

 of concentrated HC1. The addition of too much acid might 

 cause the nitrates to respond to the reaction. Add two cubic 

 centimeters of the sulfanilic acid solrtion of hydrochlorid of 

 naphthylamin, mix, cover with a watch glass, and set aside 

 for 30 minutes. Prepare at the same time other nessler tubes 

 containing known amounts of the standard solution of sodium 

 nitrite and diluted to the TOO cubic centimeter mark with 

 * Examination of water, 3rd edition, 1906 : 42. 



