THE MEASUREMENT OF pH ix«-15 



detect, more roughly, pH values over wider zones of pH. Such papers are more re- 

 liable in buffered solutions than in unbuffered ones. 



To be emphasized, is the fact that the capillary action of the paper and of the sizing 

 materials on the paper fibers may interfere, through selective sorption, with the normal 

 interaction of solution and indicator. Generally speaking, a generous time of soaking 

 of the paper for the establishment of equilibrium, seems desirable. On the other hand, 

 a standardization of the procedure may permit a short exposure (30 sec.) to yiehl re- 

 producible results, which are approximate in any case. See Kolthoff and Rosenblum 

 (1937). Indicator papers are not recommended, except when the use of indicator 

 solutions is precluded and a mere approximation is sufficient. 



TiTRATABLE AciDITY, BuFFER AcTION, AND pH ADJUSTMENT OF 



Culture Media 



In the titration of an acid with an alkah, or vice versa, a pH is 

 reached at which the number of equivalents of acid equals the number 

 of those of alkali. This pH is the equivalence point ("end-point"). 



If both the acid and the alkali are completely ionized, e.g., HCl 

 and NaOH, it is simple to calculate that this pH is about 7, and 

 that, in the case of 0.1 N reactants, the pH of the HCl solution will 

 sweep precipitously from about pH 4 to 7 upon the addition of the 

 last tenth per cent of NaOH; further, the addition of the first tenth 

 per cent excess of NaOH will cause a shift from pH 7 to about 10. 

 In other words, the titration curve, constructed by plotting pH as 

 ordinates and per cent neutralization as abscissas, is very steep at the 

 equivalence point (pH 7) in this titration. 



The ideal indicator for the detection of this equivalence point 

 would be one capable of giving a distinctive color at pH 7, e.g., brom- 

 thymol blue. In practice, however, the steepness of the titration 

 curve of the HCl at the equivalence point in the above example will 

 permit this indicator to pass sharply from yellow to blue upon the 

 final addition of a negligibly small excess of NaOH. For this reason, 

 phenolphthalein (pK' 9.7) is frequently used for this purpose because 

 the first appearance of its pink color, at about pH 8.5, is a convenient 

 and usually sufficiently accurate indication of the endpoint of such a 

 titration. 



In fact, except for refinements that may be neglected for ordinary 

 purposes, pH 8.5, detectable by means of phenolphthalein, is a fairly 

 satisfactory endpoint for the titration of strong acids and of all weak 

 acids with pK' values of less than 6.0. In the case of acids with pK' 

 values greater than 6.0, it is necessary, by application of equation 5, 

 to calculate the pH of the equivalence point, and to refine the method 

 of endpoint determination. For a discussion of the elementary 

 theory of acid-base titration, see Clark (1928). 



Titratable acidity of a culture. The titration of an acid (or a base) 

 to an equivalence point, as discussed above, is a rational application 

 of simple acid-base theory. On the other hand, in the titration of 

 complex mixtures such as milk, tissue extract, or culture media, an 

 equivalence point has no precise meaning. In such a case, the 

 selection of an endpoint pH is arbitrary, and fixed by custom (e.g., 

 pH 8.5 with phenolphthalein) or by some special requirement. 



In bacteriology, there is frequent need for determining the so-called 

 titratable acidity produced during the growth of a culture in a fluid 



