TITRATION AND ADJUSTMENT OF CULTURE MEDIA 229 



of its varying acid constituents. In the last 10 or 15 years 

 phenolphthalein has been largely employed, yet it cannot be 

 relied upon in every instance. 

 Park and Wilhams-^ state: 



Different indicators differ not only in delicacy but in the substances 

 to which they react. A medium alkaline to litmus is acid to phenol- 

 phthalein showing that there are present substances possessing a char- 

 acter which litmus does not detect, weak organic acids and organic 

 compounds, theoretically amphoteric but in which an acid character 

 predominates. 



Thus a liter of bouillon becomes, on the addition of 1 per cent of 

 peptone, more alkaline to litmus but decidedly more acid to phenol- 

 phthalein; 1000 cc. of water with 1 per cent peptone is acid to phenol- 

 phthalein to such an extent that 3.5 cc. of deci-normal NaOH is re- 

 quired to neutralize it. To litmus it is alkaline and requires 3.4 cc. 

 of deci-normal HCl. Two per cent peptone doubles the difference. 

 The same figures hold approximately true for peptone broth. 



Eyre (1915) states that although meat infusion is always 

 acid to phenolphthalein it may react neutral or even alkaline 

 to litmus; again, if rendered exactly neutral to litmus, it still 

 reacts acid to phenolphthalein; that this is due to the facts: 



(1) Litmus is insensitive to weak organic acids whose presence is 

 readily indicated by phenolphth ilein. 



(2) Dibasic sodium phosphate which is formed during process of 

 neutralization is a salt which reacts alkaline to litmus but neutral to 

 phenolphthalein . 



On the other hand, MacNeal (1914) considers litmus the more 

 useful: 



The neutral point indicated by litmus is very nearly the actual 

 point in respect to acidity and alkalinity, and this point is not appre- 

 ciably displaced in either direction by the addition of a neutral mixture 

 of a feebly dissociated acid and its salts to the solution. The end re- 

 action indicated by phenolphthalein when it turns pink is actually a 

 point at which there is a slight excess of alkali. This is so nearly the 

 neutral point in inorganic solutions, when electrolytic dissociation 



21 Third and fourth editions— 1908 and 1910. 



