486 CLARK AND LUBS : INDICATORS FOR CULTURE MEDIA 



of Hurwitz, Meyer, and Ostenberg 9 has been found useful for this pur- 

 pose. We have also developed the dilution method, which consists in 

 diluting about 2 cc. of the tested solution to 10 cc. with distilled water 

 and measuring the P H of this comparatively clear dilution. As is well 

 known, this degree of dilution of solutions such as those tested has so 

 small an effect on the P H value that it can seldom' be detected by the 

 crude colorimetric method. 



TABLE 2 



Deviations of Colorimetric from Electrometric P h Determinations of 



Beef Infusion Media 



NO. OF 

 DETER- 

 MINA- 

 TIONS 



7 

 6 

 4 

 4 

 2 



10 

 5 



14 

 5 



12 

 8 

 6 

 3 

 3 

 5 

 3 

 3 

 3 



INDICATOR 



Brom-phenol blue . . . 



Methyl red. 



Methyl red 



Propyl red 



Propyl red 



Brom-cresol purple . . 

 Brom-cresol purple. . 

 Brom-thymol blue . . 

 Brom-thymol blue . . 



Phenol red 



Phenol red 



Cresol red 



Cresol red 



a-Naphtol-phthalein 



Thymol blue 



Thymol blue 



Phenol-phthalein. . . . 

 o-Cresol-phthalein . . 



comparator 



comparator 



dilution 



comparator 



dilution 



comparator 



dilution 



comparator 



dilution 



comparator 



dilution 



comparator 



dilution 



comparator 



comparator 



dilution 



comparator 



comparator 



MINIMUM 



-0.01 

 ±0.00 

 ±0.00 

 ±0.00 

 ±0.00 

 ±0.00 

 -0.01 

 +0.03 

 +0.01 

 -0.01 

 -0.02 

 -0.01 

 -0.02 

 -0.02 

 -0.02 

 -0.01 

 -0.01 

 -0.01 



The confusing effect of turbidity has been found to be more serious 

 in many instances than the coloration usually encountered. This has 

 been especially noticeable when either brom-phenol blue or brom-cresol 

 purple was used. These indicators arc red in thick layers of their 

 solutions but blue in thin layers (at. the proper P H ) . The impossibility 

 of establishing with such indicators a good comparison between a 

 turbid solution, which can not be effectively viewed in any great depth, 

 and a clear comparison standard of the same P H is quite evident. With 



9 Hurwitz, S. H., Meyer, K. F., and Ostenberg, Z. 

 Hospital, 27: 16. 1916. 



Bull. Johns Hopkins 



