184 PA THOGENIC BACTERIA. 



" The first thing to obtain is a standard ' indicator' which will 

 give uniform results. These requirements are best fulfilled by 

 phenolphthalein. This indicator was first suggested by Schultze 

 in combination with the titration method for obtaining the desired 

 reaction for culture-media, 1 but its general adoption seems to 

 have been retarded by Dahmen, 2 who claimed that it was not 

 feasible owing to the complications that might arise from the 

 presence of carbonates and ammonium salts in the solution to be 

 tested. These objections to the use of phenolphthalein do exist, 

 but may be readily overcome. The amount of free and combined 

 ammonia present in the culture-media at the time the reaction is 

 determined has been found not to exceed 0.003 P er cent., which is 

 less than one-tenth the amount which interferes with the accuracy 

 of the indicator, while the production of carbon dioxid is obviated 

 to a very great degree by neutralizing with sodium hydroxid 

 instead of sodium carbonate, and any of this gas which may be 

 absorbed from the atmosphere is practically all driven off by the 

 heat during the preparation of the media." 



The great advantage of the use of phenolphthalien over other 

 indicators lies in the fact that it takes into account the reaction 

 of weak organic acids and of organic compounds which have an 

 amphoteric reaction, but in which the acid character predominates. 

 Turmeric possesses the same properties, but the change of color 

 from yellow to brown is less satisfactory than the development 

 of purple-red color. Furthermore, turmeric paper changes rather 

 slowly, while with phenolphthalein the color appears almost 

 instantly. Another advantage to be gained from the use of this 

 latter indicator is its behavior toward the phosphates. Petri and 

 Moussen 3 and Timpe 4 have shown that the amphoteric reaction 

 of media is associated with the presence of phosphates, and that 

 there are present in peptone and gelatin proteid bodies which 

 possess both an acid and a basic nature, but in which the acid 

 character predominates. These observers agree that to deter- 

 mine accurately the reaction of such amphoteric compounds 

 phenolphthalein (or turmeric paper) should be used as an 

 indicator. 



It is known that at the neutral point of phenolphthalein any 

 free phosphoric acid present enters into combination, and the 

 monobasic and tribasic salts of this acid are changed to the dibasic 

 form (Na 2 HPO i ). Now disodium hydrogen phosphate reacts alka- 



1 Centralbl. f. Bakt. u. Parasitenk. , 1891, Bd. x., p. 53. 



2 Ibid., 1892, Bd. xii., p. 620. 



3 Arbeiten aus dem k. Gesundheitsamte, 1893, Bd. viii., p. 311. 



* Centralbl. f. Bakt. u. Parasitenk., 1893, Bd. xiv., p. 845; 1 894, Bd. xv., 

 PP- 394-644; 1893, Bd. xvii., p. 416. 



