484 CLARK AND LUBS: INDICATORS FOR CULTURE MEDIA 



The chief object of these studies has been to assemble a set of 

 indicators which may be used from about P H = 1.0 to P H = 10.0 

 and which will have such brilliancy and such reliability that they 

 may be used in the colored and turbid solutions handled by the 

 bacteriologist. Preliminary tests had shown the value of some 

 of the indicators of the methyl red and sulphone-phthalein types. 

 We have now concluded a more extensive investigation in which 

 over four hundred electrometric measurements were made of the 

 hydrogen-ion concentrations of a variety of culture media and 

 cultures with simultaneous measurements by the colorimetric 

 method. 



In these studies we have had to test the applicability of the 

 indicators upon a heterogeneous collection of solutions such as 

 are used in bacteriological work and, in order to subject the method 

 to the severe conditions which it will have to meet if applied to 

 many bacteriological problems, we have devoted most of our 

 attention to measurements of colored and turbid solutions. The 

 material was therefore not favorable for any systematic study of 

 the so-called "protein and salt errors." Furthermore, since we 

 consider the colorimetric method to be only supplementary to the 

 more precise electrometric method, we confined our attention to 

 very simple and rapid colorimetric procedures, such as are avail- 

 able to all and such as are convenient for handling the enormous 

 number of tests which certain classes of research and routine 

 bacteriological work require. The electrometric measurements, 

 on the other hand, were made with care and with the improved 

 equipment described by Clark 3 and by Clark and Lubs. 4 



The details of these extensive comparisons between the elec- 

 trometric and the colorimetric determinations are beyond the 

 scope of this brief article. They will be published elsewhere, to- 

 gether with a discussion of the applications of the colorimetric 

 method in bacteriology. The main results may be briefly sum- 

 marized as follows. 



Since the colorimetric method, if applied extensively in routine, 

 should be made as convenient as possible, we devised and carefully 



3 Clark, W. M. Journ. Biol. Chem., 23: 475. 1915. 



4 Clark, W. M., and Lubs, H. A. Journ. Biol. Chem., 25: 479. 1916. 



