THE MEASUREMENT OF PH AND TITRATABLE ACIDITY 89 



include all the volatile acids, including CO2 and bicarbonate, that may be 

 present in the culture that is being titrated. 



It should be emphasized that in most cases, the titratable acidity is 

 merely a measure of the buffering capacity (see below) of the medium 

 within the pH range observed. It does not permit further interpretation 

 without additional data on the components of the culture. The titratable 

 acidity is of some importance, along with final pH, in the comparison of 

 high-acid-producing organisms. For such comparisons to be valid, it is 

 necessary that the different organisms be grown in the same medium. 

 Different media which vary in buffering capacity may yield misleading 

 results. 



Buffer action. The titration curve of a weak acid has a sigmoid shape, 

 each end of the curve having a large (steep) slope and the main central 

 portion having a small slope. This small slope expresses the buffer action 

 of the system, that is, the ability of the system (comprising the weak acid 

 and its salt) to resist large change in pH on the addition of acid or alkali. 

 The sigmoid shape of the titration curve expresses, therefore, the fact that 

 the buffer action of such a system is maximal at the mid-point and 

 decreases on either side of this point, first gradually and then more 

 extensively as either end of the curve is approached. The limits of the 

 pH zone of effective buffer action may be* arbitrarily set at 1.5 pH units 

 greater and less than the pK' of the acid of the buffer system. It is obvi- 

 ous that increasing the concentration of the buffer system will increase its 

 buffer action; therefore buffer action also depends upon the concentration 

 of the buffer system. 



The buffer action of a culture medium is dependent on its composition 

 and may vary considerably in different regions of pH. Significant results 

 as to final pH and titratable acidity in cultures depend to a large extent 

 on comparisons made in media having buffer action that is uniform and 

 adjusted in amount to the purpose of the test. A method for estimating 

 such buffer action is as follows : 



Assume, for example, that the initial pH of a culture medium is 6.8 and 

 that it is desired to measure the buffering capacity of the medium between 

 the pH limits 5.0 and 8.0. This can be done by titrating an aliquot, e.g., 

 5 ml, of the medium with 0.057V HCi to pH 5.0 and another aliquot with 

 0.05A^ NaOH to pH 8.0. The sum of these titers gives a simple and 

 useful measure of the buffering capacity of the medium within the pH 

 zone 5.0-8.0. Brown (1921) has described the procedure and some of its 

 practical uses. 



The pH -adjustment of a culture medium. This is done with the 

 medium at about 80-90 per cent of its final volume. Prepare approxi- 

 mately normal NaOH and HCI stock solutions and also about 100 ml of 

 each of these solutions diluted with distilled water exactly to one-tenth 



