610 LUBS AND CLARK: NEW HYDROGEN-ION INDICATORS 



Sorensen has done invaluable service by eliminating from these 

 tables the comparatively useless indicators and in his excellent 

 resume he has offered a selection of undoubted value. Unfortu- 

 nately some of these are of little value in bacteriological media 

 and others, from our experience, might be replaced with advan- 

 tage. The eliminations leave some serious gaps in the series. 

 There are, however, some indicators which have been studied 

 since Sorensen's compilation, a few of which have not been in- 

 vestigated with reference to their applicability for determining 

 hydrogen-ion concentration, and there also remain the possi- 

 bilities in new syntheses, as Sorensen has suggested. 



Walpole^ in a recent article gives an excellent resume of the 

 method and suggests some new indicators. Those suggested are 

 either not particularly satisfactory for our purpose or are rather 

 difficult to obtain or prepare. 



These considerations have led us to the investigation and 

 compilation herein recorded. A more detailed account of the use- 

 fulness of particular indicators, of their protein and salt errors, the 

 determination of their apparent dissociation constants, and our 

 attempts to systematize the method for the general use of the 

 bacteriologist will be reserved for subsequent papers. This 

 paper is to be regarded merely as a preliminary description of 

 our work in this field. 



The importance of a simple, accurate, and well-systematized 

 colorimetric method of determining the hydrogen-ion concen- 

 trations of bacteriological culture media and cultures has been 

 pointed out in previous papers^ from this laboratory. 



PREPARATION AND PROPERTIES OF INDICATORS OF THE 



METHYL RED TYPE 



Methyl Red, o-Carboxybenzene-azodimethylaniline, N(CH3)2 — C6H4 

 — N2 — C6H4COOH, was first prepared by Rupp and Loose.^ Tizard" 

 later improved the method of preparation so that almost quantitative 

 yields were obtained. He determined its apparent dissociation constant 



3 Biochem. Journ. 8: 628. 1914. 



* Journ. Infect. Diseases 17: 109. 1915; ibid. 17: 160; Journ. Biol. Chem. 22: 

 87. 1915. 



5Ber. d.deutsch. Chem. Ges. 41: 3905. 1908. 

 6 Trans. Lond. Chem. See. 97: 2477. 1910. 



