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BIOLOGY AND TECHNIQUE 

 HYDROGEN ION CONCENTRATION PH VALUE 



Indicators are substances, usually weak organic acids or 

 which change in color when subjected to changes of reaction, that is, 

 to changes of hydrogen or hydroxyl ion concentrations. A table 

 which we take from a pamphlet prepared by the Army Medical School, 

 will give the color reactions at various P# measurements for a number 

 of common indicators. 



COLOR REACTIONS IN SOLUTIONS OF DIFFERENT HYDROGEN ION CONCENTRATION (2) 



Clark and Lubs particularly have been responsible for introducing 

 the method at present in use in bacteriological laboratories, based 

 upon the preparation of solutions which can be used as standards for 

 the colorimetric measurements of culture media. The solutions must 

 be very carefully prepared, and should, whenever possible, be con- 

 trolled by potentiometer measurements; however, with proper care 



