Xanihene Dyes 201 



affected by the presence of neutral salts and proteins than are 

 many other indicators. 



The first list of sulfonphthalein indicators was published by 

 Clark and Lubs (1917), who to make their series complete had to 

 include methyl red, an azo dye, less satisfactory because its solu- 

 tions are subject to reduction on standing. More recently Cohen 

 (1923, 1926) added six new sulfonphthalein indicators, among 

 them two (brom cresol green and chlor cresol green) which have 

 sensitive ranges so close to methyl red as to make the latter 

 unnecessary. 



These indicators, unlike the usual dyes, are ordinarily furnished 



in the form of color acids, which are practically insoluble in water. 



They must accordingly be employed in alcoholic solution or must 



be converted into the di-sodium salt with the proper amount of 



NaOH, the quantity of the latter to use being figured by the 



following formula: 



TTT • 1 . ^- /-wTT Weight indicator X 40 



\\ eight ^ aOH = ^ ■,.., • — 7—' 



Molecular weight indicator 



The alcoholic solutions are simpler to prepare and for ordinary 

 indicator purposes are equally satisfactory; they cannot, of course, 

 be employed, in vital staining. 



Table 3, above, quoted from Leaflet IX of the Manual of 

 Methods for Pure Culture Study of Bacteria (Soc. of American 

 Bacteriologists; published by Biotech Publications, Geneva, 

 N. Y.), shows the color changes and sensitive ranges of the in- 

 dicators described below. (This table also includes methyl red, 

 which is an azo dye, and is described in Chapter V of this book.) 



In the following list the compounds are arranged in the order 

 of their dissociation constants and hence in that of their sensitive 

 ranges in the pH-scale. 



»5 META-CRESOL PURPLE 



pK = 1.5, 8.3 



This indicator is 7?i-cresol sulfonphthalein. 



HO OH 



/ C \ 

 CH3 / -O CH3 



so, 



An indicator changing from red to yellow in its acid range and 

 from yellow to purple in its alkaline range. 



