HOOKER'S METHOD 457 



of acetic acid. The exact strength of this solution, is of no 

 material importance to the success of the process, and the above 

 proportions have been selected principally because they are con- 

 venient. The solution will remain unchanged for several months. 

 The use of this solution merely facilitates the prepara- 

 tion of that next described, which will not keep, and has, conse- 

 quently, to be freshly prepared for each series of determinations. 



3. The sulfuric acid solution of carbazol is easily made in a 

 few seconds, but it is advisable to allow it to stand from one and 

 one-half to two hours before using. It is prepared by rapidly 

 adding 15 cubic centimeters of sulfuric acid to one cubic centi- 

 meter of the above described acetic acid solution. This quan- 

 tity usually suffices for from two to three nitrate estimations. 

 When freshly prepared it is golden-yellow or brown; it changes 

 gradually, however, and in the course of one and one-half or 

 two hours it becomes olive-green. This change is probably due 

 to traces of oxidizing agents, which occur in the sulfuric and 

 acetic acids, and which, although not present in sufficient quan- 

 tity to act immediately, gradually bring about the reaction de- 

 scribed. The greenish color does not interfere with the process, 

 as might at first be supposed ; on the contrary, the solution is not 

 sensitive to small quantities of nitric acid until it has undergone 

 the change to olive-green, and it is for this reason that it should 

 be prepared about two hours before required for use. This solu- 

 tion may be thoroughly depended on for six hours after prepara- 

 tion. The intensities of color produced by the more concentrated 

 solutions of nitrates after this time gradually approach each other 

 and become ultimately the same. 



4. The standard solutions of potassium nitrate are very readily 

 prepared. The solutions which are to be compared directly with 

 the waters examined, may be prepared as required, but if many 

 determinations are to be made with a variety of samples, it will 

 be found best to prepare a complete series, differing from each 

 other by 0.02 part nitrogen in 100,000. This series may include 

 solutions containing quantities of nitrogen in 100,000 parts, rep- 

 resented by all the odd numbers from 0.03 up to 0.39. It will 

 be found convenient to prepare them in quantities of 100 cubic 



