REACTION AS DETERMINED BY TITRATION 113 



REACTION AS DETERMINED BY TITRATION. 



In the development of this method, it is of prime impor- 

 tance that the indicator used shall announce as nearly as 

 possible the true neutral point, i. e., neutral as represented 

 by distilled water. Experience with three well-known indi- 

 cators will illustrate the importance of settling this point: 

 If, for instance, we decide to establish the reaction of a 

 volume of meat infusion agar-agar, with the view of ulti- 

 mately adjusting that reaction to neutral, we shall first 

 find (as with practically all other artificial media) that the 

 mass is acid. If we now undertake to determine the volume 

 of T KOH solution that is necessary to neutralize the acid 

 present, we shall find that volume to vary considerably 

 according to the indicator employed. If phenolphthalein 

 be selected as the indicator, about 47 c.c. of the alkaline 

 solution per liter of culture medium will be required; whereas, 

 if litmus be used instead of phenolphthalein, only about 

 28 c.c. of the alkali will be needed, while if rosolic acid be 

 substituted for litmus the figure for the alkali required falls 

 to about 5 c.c. Obviously, if our information ceased here 

 we would be at a loss to know just which of the three titra- 

 tions represented exact neutralization of the mass. 



Fortunately, the application of precise physical methods 

 to the study of indicators has established their relative 

 values and revealed the limits of their usefulness under 

 various circumstances. As a result of such studies we are 

 informed that the neutral point (the reaction of pure dis- 

 tilled water) as announced by phenolphthalein is a little 

 high, while that given by rosolic acid is so low as to be 

 altogether misleading. On the other hand, the indications 

 afforded by litmus may be exact or nearly so, but as litmus 



