202 Biological Stains 



slO THYMOL BLUE 



pK=1.5, 8.9 

 This is thymol sulf onphthalein : 



CH3 CH3 CH3 CH3 



CH CH 



HO I I OH 



\/\ 



CH3 c _ (:h3 



/ o 



so. 



Thymol blue is now a well known indicator, used both in its acid 

 range (red to yellow) and in its alkaline range (yellow to blue). 

 In the latter range it may well be replaced with meta-cresol purple. 



si 5 BROM PHENOL BLUE 



pK = 4.0 



This is tetra-bromo-phenol sulfonphthalein. 



Br 



OH 



An indicator changing from yellow to blue. 



«20 BROM CHLOR PHENOL BLUE 



pK = 4.0 

 This is di-bromo-di-chloro-phenol sulfonphthalein. 



CI 



OH 



An indicator almost identical with the last in range, and showing 

 the same change in color. 



