234 BERTHA VAN H. ANTHONY AND C. V. EKROTH 



evidence desired in every case, e.g., the differences between 

 litmus and phenolphthalein. 



The necessary dependence on the change in color of an indi- 

 cator to show the reaction of a solution, especially in the case 

 of culture media, gives rise to error. This is due partly to the 

 depressing effect of colloids, phosphates and some sugars which 

 affect the sensitiveness of indicators; and also to variations of 

 judgment with different workers as to the correct shade of 

 color for an end point. Where it is desirable to avoid these 

 influences titrations may be made by measurement of the elec- 

 trical potential. 



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Abbott, A. C. 1915. Principles of Bacteriology, 9th ed., p. 109. 



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Clark, W. M. 1915. Reaction of bacteriologic culture media. Jour, of Infect. 

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Committee on Standard Methods of Water Analysis. 1905. A. P. H. A. 

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Committee on Standard Methods of Water Analysis. 1913. A. P. H. A., 

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