558 J. HOWARD BROWN 



standard acid or alkali from one known hydrogen ion concentra^ 

 tion to another. In the determination of such an index two 

 factors must be more or less arbitrarily selected, the limits of 

 hydrogen ion concentration between which titration is to be 

 performed, and the acid or alkali to be used. 



Clark (1915b), Bovie (1915), and Clark and Lubs (1917) have 

 published titration curves of bacteriological media. From 

 these curves it is seen that if one starts with a medium of pH 

 8.0 or a little more alkaline and titrates with hydrochloric acid 

 to pH 5.0 the curve is practically a straight line. If a weak 

 acid such as lactic or acetic acid is used the curve begins to 

 flatten out slightly between pH 6.0 and pH 5.0, and markedly 

 after leaving pH 5.0 because of the lower dissociation constant 

 of the weak acids. The greater part of the range of hydrogen 

 ion concentration of bacteria of interest to the pathologist and 

 sanitarian lies between pH 8.0 and pH 5.0. This is also the 

 range of NaoHP04;=iNaH2P04. The acids formed in cultures 

 are mixtures of weak acids but within the range mentioned the 

 curves of acetic and lactic acid are ahnost identical with that of 

 hydrochloric acid. It would seem therefore that for the general 

 purpose of determining the relative buffer values of media hy- 

 drochloric acid may well be employed. 



The ''buffer indices" of a number of samples of bouillon from 

 various laboratories have been titrated. The method has been 

 (1) to determine the hydrogen ion concentration of the medium, 

 which usually lies between pH 7.0 and pH 8.0, (2) to add suffi- 

 cient N/20 NaOH from a burette to reduce the reaction of the 

 sample to pH 8.0, and then (3) to the same or another sample 

 sufficient N/20 HCl to raise the hydrogen ion concentration to 

 pH 5.0. The amount of alkali required to reduce the hydrogen 

 ion concentration of a medium from its initial reaction to a 

 stated lower hydrogen ion concentration, say pH 8.0, may be 

 called the ''reserve acidity" (Washburn 1910) of the medium^ 



^ It is to be noted that the terms "reserve acidity," "reserve alkalinity" and 

 "buffer index" are qualified by the pH values between which the titrations are 

 made. While the determinations here reported are for BI (pH 8-5), for certain 

 problems it may be advisable to adopt other limits of hydrogen ion concentration, 

 as for instance BI (pH 9-5) or BI (pH 5-3). 



