124 LABORATORY BACTERIOLOGY 



weak organic acids and of organic compounds which have an am- 

 photeric reaction, but in which the acid character predominates. 

 Turmeric possesses the same properties, but the change in color from 

 a yellow to brown is less satisfactory than the development of 

 purple red color, and furthermore turmeric paper changes color 

 rather slowly, while with phenolphthalein the color appears almost 

 instantly. 



Another advantage to be gained from the use of this later indi- 

 cator is its behavior toward the phosphates. Petri and Maassen 

 (Arbeiten aus dem K. Gesundheitsamte, Bd. VIII., 1893, S. 311) 

 and Timpe (Cent, fur Bakt. und Parasit., Bd. XIV., 1893, S. 845; 

 Bd. XV., 1894, S. 394-664; Bd. XVII., 1893, S. 416) have shown 

 that the amphoteric reaction of media is associated with the pres- 

 ence of phosphates, and that there are present in peptone and gela- 

 tin proteid bodies which possess both an acid and a basic nature, 

 but in which the acid character predominates. These observers 

 agree that to determine 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 mono- 

 basic and tribasic salts of this acid are changed to the dibasic form 

 (Na2HP04). Now disodium hydrogen phosphate reacts alkaline 

 to litmus, lacmoid, rosolic acid, and methyl-orange, but neutral to 

 phenolphthalein and turmeric. 



Studies made at the Lawrence Experiment Station show that this 

 acid salt may be added to culture media in amounts greatly exceed- 

 ing those naturally present in the media without producing any 

 apparent influence upon bacterial development. 



From these facts it seems clear that the use of any of the above- 

 mentioned indicators, other than phenolphthalein and turmeric, in 

 the presence of this dibasic phosphate, prevents the addition of a 

 sufBcient amount of free alkali to effect neutralization, and as the 

 amount of phosphates in media varies considerably, the reaction 

 passes beyond accurate control when litmus and other substances of 

 its class are used as indicators. 



Datum point to which all degrees of reaction shall be referred : 



From the available evidence it seems advisable to adopt the phe- 

 nolphthalein neutral point as the fixed point to which all degrees 

 of reaction shall be referred. 



