HYDROGEN IONS, ETC., OF BACTERIOLOGICAL MEDIA 567 



Many organisms grow less abundantly in fermented bouillon 

 plus glucose than in unfermented bouillon plus glucose although 

 the two media have equally high buffer indices. Nevertheless, 

 the buffer index is one of the most important factors and one 

 which should be determined. ^ 



CONCLUSIONS 



The titration of media is not to be regarded as a crude method 

 of determining the reaction of media but a process which reveals 

 facts not disclosed by a simple hydrogen ion determination. 



For many common purposes a knowledge of the buffer content 

 of media is quite as important as the hydrogen ion concentration. 



The buffer content between stated limits of hydrogen ion 

 concentration is easily defined as the buffer index which is the 

 sum of the reserve acidity and reserve alkalinity between those 

 limits. 



A simple colorimetric method of determining these values is 

 described, a method which need not consume more than five 

 minutes time. The determination can be made by any labor- 

 atory helper who can make a titration or a hydrogen ion deter- 

 mination and should be recorded for each lot of medium made. 



There is appended a copy of instructions for laboratory helpers 

 and a convenient form of record on which is recorded a sample 

 titration. 



The author wishes to acknowledge his indebtedness to Dr. 

 P. E. Howe of this department for valuable suggestions and 

 criticisms. 



^ Since this paper has been written there has appeared the paper on The Rela- 

 tion of Hydrogen-ion Concentration to the Growth, Viability, and Fermentative 

 Activity of Streptococcus hemolyticus by L. F. Foster (Jour. Bact., March, 1921, 

 6, 161). The author illustrates admirably some of the points brought out in the 

 present paper. He emphasizes the necessity of knowing the buffer content of a 

 medium. 



