172 



the proteoses will be in a large measure precipitated out if the acidity be- 

 comes greater than Ph 5.4, while the precipitating point for the phosphates 

 is around Ph 8.8. 



In an effort to determine just what the variations in the final result 

 might be, between the use of the two methods, a number of samples of 

 media were made up according to the American Public Health Association 

 standards and the reaction determined by both methods. The procedure 

 they laid down was used in making the media +1, and the colorimetric 

 method, using the indicators and standards of Clark and Lubs, was fol- 

 lowed in determining the hydrogen ion concentration. 



From the accompanying table it may be seen that the actual amount 

 of base necessary to make the media neutral was less than that indicated 

 by the phenolphthalein titration in all cases except the bouillon made up 

 with Wittes Peptone. Also the results of this experiment show that the 

 media made up with Difco Peptone was practically neutral as made, how- 

 ever due to the buffer effect of the peptone the phenolphthalein titration 

 indicated that it was necessary to add base. In the case of the agar media 

 where two per cent peptone was used, the difference between the actual 

 amount of base necessary to bring the media to neutrality and the amount 

 indicated by the phenolphthalein titration is greater than in the bouillon. 

 In the gelatin media the bouillon was neutralized before the gelatin was 

 added and as a result there is no difference indicated between the two pep- 

 tones, however, there is a great dift'erence shown between the two methods 

 of determining the reaction. 



The results of these tests show that the method of making media +1 to 

 phenolphthalein can not be depended upon to bring it to neutrality and that 

 in the case of all careful work the reaction must be determined by measure- 

 ment of the hydrogen ion concentration. 



References. 



Clark, Wm. M. and Lubs. H. A. 



"The Coloi-imetric Determination of the Hydrogen Ion Concentration 



and Its Applications in Bacteriology."' 



Jour. Bact. 1917, Vol. 2, 1-.34, 109-130, 191-236. 

 Kligler. I. J. 



"The Effect of Hydrogen Ion Concentration on the Production of 



Precipitates in a Solution of Peptone and Its Relation to the Nutritive 



Value of Media." 



Jour. Bact. 1917. Vol. 2, 351-353. 

 Cohen, B. and Clark, Wm. M. 



"The Influence of the Ph of Media upon the Reproduction of some 



Common Bacteria." 



Proceedings of the Amer. Soc. Bact. 1917. 

 Clark. Wm. M. 



"The Adjustment of the Reaction of Culture Media." 



Proceedings Amer. Soc. Bact. 191G. 



