CLARK AND LUBS I INDICATORS FOR CULTURE MEDIA 489 



solutions we have studied as the two-colored indicator thymol blue. In 

 our experience a two-colored indicator is generally to be preferred for 

 hydrogen-ion determinations, especially when the solution itself is 

 colored. On the other hand these phthaleins can be used at slightly 

 higher P H than thymol blue and consequently must be used in certain 

 instances. Of the two we prefer the cresol compound, because of its 

 greater brilliancy. Indeed we now use the cresol compound in place 

 of phenol-phthalein in ordinary titrations. 



Determinations with whey, banana juice, unfiltered extract of cow 

 feces, thick green silage juice, overheated bouillons containing sugar 

 decomposed to a dark brown solution, and vegetable extracts such as 

 that of the potato which had oxidized till it appeared perfectly black 

 in bulk are samples of the material we have handled with errors in P H 

 which seldom were as great as 0.3 Ph and generally much less. 



A consideration of certain broad principles involved in bacteriologi- 

 cal studies has led us to believe that in general the order of accuracy 

 which may be attained with these indicators when using simple and 

 rapid procedures is quite adequate for the testing of acid and alkali 

 fermentations, for the study of the effect of P H upon the stability or 

 decomposition of culture media, for studying the effect of P H upon the 

 filterability of toxins, enzymes, etc., for determining the effect of P H 

 upon bacterial metabolism in general and enzyme activity in particular, 

 and for controlling the reaction of solutions during the study of various 

 processes. One of us 11 has already called attention to the inadequacy 

 of the titrimetric method of adjusting the so-called degree of reaction 

 of bacteriological culture media. For adjusting to various P H values 

 the indicators are quite adequate. 



It may be mentioned that many of the criticisms which we have 

 urged against the use of titration methods by the bacteriologist apply 

 with equal force to many tests of the so-called titratable acidity of 

 natural products or extracts thereof. To maintain that the analytical 

 content of acid in some of these solutions can be determined by titra- 

 tion to a given tint of phenolphthalein is untrue. The differences which 

 are determined and which often are of great practical value may fre- 

 quently be observed with greater clarity by colorimetric P H determina- 

 tions. The indicators we have described should therefore be useful in 

 a wide variety of instances. 



11 Clark, W. M. Journ. Infect. Diseases, 17: 109. 1915. 



