HYDROGEN ION CONCENTRATION AND PROTEOLYSIS. 161 



So far as the macroscopical examination was concerned abundant and 

 uniform growth took place in each lot. 



Medium IV., having satisfied the chemical, physical and biological 

 tests better than Medium 1., II. and III., and being in regular use in 

 microbiologic work, it appeared as suitable as anj^ for the investigation 

 under way. 



Preparation oj Media of Different Hydrogen Ion Concentration.. 



In the previous experiment Medium IV. was found to be satisfactory 

 so far as it had been in\ estigated. It is now necessarj^ to adjust its reaction 

 by the addition of acid or alkali, so that a series of the medium of various 

 hydrogen ion concentrations, viz., Ph = 1 to 13, may be obtained. Only 

 thus can it be employed to accomplish the purpose of the proposed inves- 

 tigation. 



It is not an easy task, however, to secure such a series of hydrogen ion 

 concentrations in such a medium, because the presence of amphoteric 

 substances and mechanical factors involved in the course of preparation 

 should be taken into consideration. First, to obtain the approximate 

 amount of acid or alkali to be added the colorimetric method was resorted 

 to. 



The Colorimetric Method. ^ — On the one hand is the medium, on the 

 other the standard solution of known liA'-drogen ion concentration. An 

 organic dye that changes color with a variation in the amount of hydrogen 

 ion concentration is employed as an indicator. The indicator is added to 

 the standard solution, as well as the sample medium for comparison of 

 colors. An estimate of the hydrogen ion concentration is made possible 

 by this means. 



Concretely: To ascertain the hj^drogen ion concentration, 5 drops of 

 methyl red^ were added to 10 cubic centimeters of Medium IV. A yellow- 

 ish green color was observed. Since methyl red gives this color in a solu- 

 tion in which hydrogen ion concentration is greater than Ph = 6, this 

 medium must contain more than Ph = 6. To make the determination more 

 exact, several standard solutions were prepared, and the colors produced 

 with the indicator were traced to identical shades alike in standard solution 

 and medium. Mixing m/15 KH2PO4 and m/15 Na2HP0 in various 

 proportions, as in the table below, the different hydrogen ion concentra- 

 tions ranging from Ph =6.24 to Ph = 7.35 were obtained. 



1 Sorensen, S. P. L. Compt. rend, du Lab. de Cadsberg, 8, 1, 1909, 67. 



2 Prepared according to S. P. L. Sorensen. 



