REACTION BY HYDROGEN-ION CONCENTRATION 115 



On the average the neutral point as established by this 

 method requires for a liter of nutrient, meat infusion agar- 

 agar the addition of 47 c.c. T KOH and for a liter of meat 

 infusion gelatin 56 c.c. T KOH. Experience shows that 

 media neutral to phenolphthalein are somewhat too alkaline 

 for the best development of most bacteria. It is desirable 

 therefore to make certain corrections. In Fuller's experi- 

 ence the degree of deviation from phenolphthalein neutral- 

 ity that insures in general the best results is represented by 

 from 15 to 20 of his scale that is, to say there should 

 remain enough uncombined acid in a liter of the finished 

 media to require a further addition of from 15 to 20 c.c. 

 T KOH. Thus for instance if, as in our foregoing calcula- 

 tion 55 c.c. T KOH were needed to bring the mass to 

 phenolphthalein neutrality, we would actually add from 35 

 to 40 c.c., i. e., from 15 to 20 c.c. less than was indicated by 

 the titration. 



REACTION AS DETERMINED BY HYDROGEN-ION 

 CONCENTRATION. 



This is based on Arrhenius's demonstration that when 

 acids, bases and salts are dissolved in water their molecules 

 are dissociated into electropositive and electronegative 

 ions. Not all the molecules of such electrolytes are always 

 so dissociated, the proportion being dependent upon a 

 number of circumstances, such as degree of dilution, 

 temperature, character of solvent, etc. By appropriate 

 electrical methods the degree of such dissociation may be 

 accurately discovered. Analyses made in this manner have 

 afforded results of fundamental importance to an understand- 

 ing of solutions, of acidity, of alkalinity and of relative 

 combining values of dissolved substances. 



