161' 



(F) Workers in physical chemistry have uetermined that for each 

 acid there is a dilution beyond which the per cent ionized remains con- 

 stant. When 25 cc. of media that is, at most, only slightly acid is further 

 diluted with carbon dioxide free water (as must be done to titrate at 

 room temperature) the per cent acid ionized has reached its limit. The 

 difference between the value obtained with the hydrogen electrode and 

 that obtained by titration under proper conditions is thus small or 

 negligible. 



(G) Itano (5) (19) has found that proteolysis is optimum when 

 the hydrogen ion concentration of media is in or at the range where 

 phenolphthalein titrations properly carried out would indicate neutrality. 



Different investigators have suggested brom thymol blue and phenol 

 red for phenolphthalein. This has not been done because the paper is 

 intended to bring out errors in making media which must be corrected 

 if any indicator is used. The values used at present for the contact 

 potential prevent one from adopting any shade of any indicator as ab- 

 solute neutrality. 



The author wishes to make acknowledgment to Dr. Redfield of the 

 Bureau of Chemistry for criticisms and suggestions. Acknowledgments 

 are also due to Director C. G. Woodbury, for it is only with his consent 

 that the writer can devote any time to consideration of this subject. 



Bibliography. 



(1) Clark, Wm. Mansfield: 



1915 Journal of Infectious Diseases, Vol. 17, page 109. 



(2) Clark, Wm. Mansfield: 



1915 Journal of Infectious Diseases, Vol. 17, page 131. 



(3) Clark, Wm. Mansfield, and same author with H. A. Lubs. 



1916 Papers presented at Meeting of American Society of Bac- 

 teriologists, December, 1916, abstracted in Bacteriological Ab- 

 stracts, Vol. 1, No. 1. 



(4) Clark, Wm. Mansfield, and Lubs, H. A. 



1917 Journal of Bacteriology, Vol. 2, Nos. 1 and 2. 



(5) Itano, Arao: 



1916 Massachusetts Agricultural Experiment Station Bulletin 

 No. 167. 



11—11994 



