556 J. HOWARD BROWN 



exception must be taken. The inaccuracies of the titration of 

 media, aB it has been commonly practiced in the past, are well 

 known and it is agreed that the reaction of media should be 

 adjusted to certain hydrogen ion concentrations. Nevertheless 

 the committee's condemnation of titration seems entirely too 

 sweeping and therefore misleading. 



When one takes a sample of medium and determines how much 

 acid or alkali must be added to bring it to a certain hydrogen 

 ion concentration he performs a titration, though he may choose 

 a better indicator than phenolphthalein and may determine 

 the end point by comparison with a color standard in a compara- 

 tor block, or may determine the end point electrometrically. 

 If one wishes to determine the reaction of a culture, he must 

 make a hydrogen ion determination, but if he wishes to determine 

 ''the amount of acid produced by an organism" he must titrate 

 the culture with a strong alkali, precisely because in a well buff- 

 ered medium much of the acid formed enters into combination 

 with buffer substances and is not revealed by a hydrogen ion 

 determination. 



It has been claimed by H. M. Jones (1920b) that various 

 factors may influence the final hydrogen ion concentration of a 

 culture. Similar conclusions were reached by F. S. Jones (1920^ 

 who regards titration under well controlled conditions as quite 

 as satisfactory as the method of determination of hydrogen ion 

 concentration for the study of the fermentative activity of strep- 

 tococci. The first mentioned author states that ''the amount 

 of glucose which a given organism can consume is influenced by 

 the buffer content of the medium . . . which aids in 

 holding the concentration of hydrogen ion from the toxic limit, 

 thus permitting a larger amount of sugar to be decomposed." 

 It follows that in a poorly buffered medium the fermentation 

 of very little glucose is required to raise the acidity to the toxic 

 limit and that therefore in such a medium the presence of a 

 small amount of glucose, as an impurity in a test substance may 

 be a very disturbing factor. The possibility is illustrated by 

 the following experiment. 



