CLARK AND LUBS I INDICATORS FOR CULTURE MEDIA 485 



studied a new set of standard buffer solutions which has several advan- 

 tages over those formerly used. The details of this part of our investi- 

 gation have recently been published. 5 



A new set of indicators has been assembled. Each of them has been 

 studied sufficiently to enable us to make a selection of the most promis- 

 ing. The selection is listed in table 1, together with the short names 

 we suggest for laboratory parlance. In this table are included the 

 apparent dissociation constants. These constants, which are, of course, 

 not the true dissociation constants, were determined by the method 

 of Salm. 6 While they are only approximate, they are probably accurate 

 enough to be used by those who may wish to apply them to titrimetric 

 problems (see Bjerrum). 7 We have used them in our more detailed 

 paper merely to illustrate some points in the discussion and to deter- 

 mine the approximate theoretical limits of P H within which the several 

 indicators may be used. The limits so found are in substantial agree- 

 ment with those found empirically. They are given in table 1. 



TABLE 1 



Selection of Indicators 



The confusing effect of the natural color of most culture media, 

 vegetable extracts, etc., can be overcome to a large extent by using 

 brilliant indicators such as the sulphone-phthalein indicators are, and 

 by using the compensation method of Walpole. 8 The simple comparator 



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



6 Salm, E. Zeitschr. physik. Chem., 57: 471. 1906. 



7 Bjerrum, N. Sammlung chem. u. chem. -tech. Vortrage, 21: 1. 1915. 

 'Walpole, G. S. Biochem. Journ., 5: 207. 1910. 



