488 CLARK AND LUBS : INDICATORS FOR CULTURE MEDIA 



Propyl red we have used chiefly to cover a zone between the ranges 

 of methyl red and brom-thymol blue. This zone may now be studied 

 with the aid of brom-cresol purple. The latter indicator is "dichro- 

 matic" like brom-phenol blue, but lends itself well to use in the screened 

 light. It may be noted especially that this indicator is useful in that 

 P H zone within which the reactions of most urines fall. 



TABLE 3 



COLORIMETRIC AND ElECTROMETRIC DETERMINATIONS OF THE P H OP URINES 



INDICATOR 



Methyl red. 



Propyl red. 



Brom-cresol purple. 



Brom-thymol blue. 



Phenol red. 



With both thymol blue (alkaline range) and its dibrom derivative 

 discrepancies between the colorimetric and electrometric determina- 

 tions appear more like uncertainties' in judgment than like consistent 

 errors. In general, good agreement was found, but it is the unfavorable 

 nature of the colors which we believe may lead to error. 



Phenol red and cresol red are undoubtedly the most reliable indica- 

 tors of the series. 



In regard to phenol-phthalein we may say in the first place that 

 neither it nor its homologue, ortho-cresol-phthalein, is as useful in the 



