Hydrogen ions,titration and the buffer index of bacteriological media

Abstract

The titration of bacteriological media should not 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 puposes a knowledge of the buffer content of media is quite as important as the hydrogen ion concentration. The importance of the buffer content of media has been indicated by Kligler, 1 Bermann and Rettger, 2 Bronfenbrenner and Schlesinger, 3 H. Jones, 4 L. F. Foster 5 and C. G. L. Wolf. 6

The buffer content of media between stated limits of hydrogen on concentration is easily determined by titration against a standard acid or alkali solution. 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” 7 of the medium indicated by the symbols Rh(PHn − 8) in which n = the initial PH. The amount of acid required to raise the hydrogen ion concentration from PHn to, say, PH 5.0 may be called the “reserve alkalinity’ indicated by the symbols R _oH(PHn − 5). The “buffer index” indicated by the symbols BI (PH8 − 5) is the sum of the reserve acidity plus the reserve alkalinity. Each of these values is to be expressed in terms of per cent. normal acid or alkali, i.e., the number of cubic centimeters of N/I acid or alkali required to change the hydrogen ion concentration of 100 c.c. of medium from one stated hydrogen ion concentration to the other. While for most purposes of interest to the sanitary or medical bacteriologist the range of hydrogen ion concentration between the limits of PH 8.0 and PH 5.0 is sufficient, the buffer index between other limits may be determined for special purposes.