612 LUBS AND CLARK: NEW HYDROGEN-ION INDICATORS 



method of Tizard for the analogous methyl red. It was recrystallized 

 from alcohol. Color changes occur over the range Ph 4.25 to P^ 6.00. 



Diethyl Red, o-Carhoxyhenzene-SiZodiQthylamyme. — This substance was 

 prepared from anthranilic acid and diethyl aniline by the method pre- 

 viously described. It was recrystallized from alcohol. Its colors are 

 slightly deeper than those of methyl red and the changes occur over the 

 range Pj^ 4.50 to P^ 6.50. 



Monopropyl Red, o-Carboxybenzene-azomonopropylaniline. — This in- 

 dicator was prepared from anthranilic acid and monopropyl aniline. 

 Color changes are from yellow to red and extend over the range Ph 4.25 

 to F^ 6.25. 



Dipropyl Red, o-Carboxybenzene-azodipropylaniline. — Attempts to 

 prepare this substance in the manner described by Tizard for methyl 

 red were not successful, so the following procedure was adopted : Five 

 grams of anthranilic acid were dissolved in two molecular equivalents of 

 3N hydrochloric acid. To the cooled solution was added the necessary 

 amount of sodium nitrite solution. Starch-potassium-iodide paper was 

 used as the indicator. Thediazotized solution was allowed to stand in ice 

 water for one-half hour and then 6.6 grams of dipropyl aniline dissolved 

 in 150 cc. of alcohol were added. The mixture was allowed to stand 

 in the ice water for two hours, and then at room temperature over 

 night. The next morning the bluish purple crystals were filtered off 

 with suction, washed with a little cold alcohol and then with water. 

 The yield, after drying in the air, was about 3 grams. Color changes 

 are from reddish-violet to yellow and occur over the range Ph4.50 

 to Ph 6.50. They are slightly more intense than in the corresponding 

 diethyl compound. 



Dimethyl-a-naphthylamineRed,o-Carhoxyhenzene-a,zodmiethyl-a-naph- 

 thylamine. — Howard and Pope^ first made this indicator. Their 

 method of preparation was used and an almost quantitative yield was 

 obtained. The color changes are extremely brilliant from purple to 

 yellow and occur over the range Ph 4.50 to Ph 6.50. 



The phenyl-a-naphthylamine, a-napthylamine, and diphenylamine 

 derivatives of this series were also prepared by Howard and Pope. 

 We investigated the two latter compounds and found that the color 

 changes were not very brilliant and probably would be of little use in 

 bacteriological work. 



The color changes of o-Carboxybenzene-azodimethyl-c^-naphthylam- 

 ine take place between Ph 5.00 and Ph 6.75. The changes of the 



9 Trans. Lond. Chem. Soc. 99: 1333. 1911. 



