120 BACTERIOLOGY 



method that the gradations of its pH are known, it is the 

 "standard" solution with which solutions of unknown pH 

 (or H.I.C.) are to be compared when the estimations are 

 made by the colorimetric method. 



Thus, if to a standard solution of pH6 we add the indi- 

 cator which exhibits its most trustworthy color changes at 

 or about that point, changing to one color or shade as we 

 approach the alkaline side, i. e., as its pH ascends, and to 

 another color or shade as we approach the acid side, i. e., 

 as pH descends, it is observed that if such indicator behave 

 in an identical manner with an unknown solution, the pH 

 of that solution is probably the same or approximately the 

 same as that of the standard solution. 



fV 



By a creful study of indicators it is found, as said, that 

 for various pH values different indicators must be chosen, 

 as none act equally well throughout all dilutions. 



According to Clark and Lubs, Medalia and others : 



Thymol blue, acid range, operates best between pH1.2 

 and 2.8. 



Brom phenol blue, between pH3 and 4.6. 



Methyl red, between pH4.4 and 6. 



Brom cresol purple, between pH5.2 and 6.8. 



Brom thymol blue, between pH6 and 7.6. 



Phenol red-, between pH6.8 and 84. 



Cresol red, between pH7.2 and 8.8. 



Thymol blue, alkaline range, between pH8 and 9.6. 



Method of Barnett and Chapman. As it is desirable for 

 routine bacteriological work to have culture media at 

 or about the neutrality of distilled water, pH7, it is 

 obvious that only that indicator which operates best at 

 or about that point is the one of most immediate inter- 

 est. Viewing the question from this standpoint Barnett 



