THE MEASUREMENT OF PH AND TITRATABLE ACIDITY 87 



instead of four, so that a tube of the unknown solution (without indicator) 

 can stand behind the pair of tubes of the standard. The tube of the 

 unknown for comparison with the standard should contain the same 

 amount of indicator as the sum of those in the two standard tubes, i.e., 

 10 drops per 5 ml; and, of course, this tube must be backed by two tubes 

 of water to equalize the optical path through the standard pair. 



Indicator papers. Passing mention may be made of these laboratory aids for the 

 approximate measurement of pH. Red and bhie litmus papers for the detection of 

 alkalinity and acidity are well known. Papers impregnated with other indicators, 

 singly or in various combinations, can be made or obtained on the market. Those 

 with a single indicator may be of use to detect roughly (about ±0.3 to 0.4 pH) values 

 within a relatively narrow zone of pH; those with indicator combinations enable one to 

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

 reliable 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 yield 

 reproducible 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 alkali, 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 O.IA^ reactants, the pH of the HCl solution will sweep precipi- 

 tously 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 neutraliza- 

 tion 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., bromthymol 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 fre- 



