TITRATION AND ADJUSTMENT OF CULTURE MEDIA 231 



alkali upon the electrical conductivity of the particular medium. 

 Such measurement should be made by means of the hydrogen 

 electrode. The measurements of potential are usually expressed 

 in terms of hydrogen ion concentration, that is, acidity. 



For an apparatus as described by Bovie (1915) the details 

 of manipulation are as follows: A mixture of the medium with 

 water in the same proportions as used for ordinary titrations 

 is placed in a beaker kept at 30°C., and the precaution of ex- 

 cluding CO2 observed. The indicator is added and the stand- 

 ard electrodes are immersed. Successive portions of deci- 

 normal sodium hydroxid solution are then added. After each 

 addition of the alkali the potential of the mixture is observed 

 and recorded. 



The point at which the indicator gives its first change of color 

 is marked. The additions of alkali should be continued and 

 the potentials further noted until the curve changes its shape — 

 that is, from concave to convex or vice versa. The actual 

 point at which this change takes place is known as the point 

 of inflection and is a true neutral point. The nearness of 

 the indicator's point of change to this point of inflection de- 

 termines its suitability for this particular class of media. The 

 indicator in which the change comes nearest to this point should 

 be selected for practical use. Work in choosing such indicators 

 according to this method is planned. 



When titrations are performed under ordinary circumstances 

 at the boiling point, it is noticeable that making a decision as 

 to the correct end point to phenolphthalein is much more diffi- 

 cult than when the temperature of 20° to 30° C. is used. This 

 is due to the changes of ionization in the mixture caused by this 

 considerable change in temperature. 



Besides this, the constant presence of colloidal substances" 

 in peptones, 23 phosphates and sugars in all media gives rise to 



22 Hildebrand (1913), Clark (1915) and others have shown that colloidal sub- 

 stances affect the sharpness of indicator end points. 



" In the titration of the peptone solutions it was very difficult to get a sharp 

 end point even at 20°C. The indicator (phenolphthalein) seemed to "flare" 

 suddenly from the colorless state to a deeper pink than the one showing a really 

 delicate end point. 



