Xanthene Dyes 205 



It is a very valuable indicator because of its dissociation constant 

 at close to true neutrality. It is yellow in acid, green in neutral, 

 and blue in alkaline solutions. It has also been employed as a 

 vital dye by McMaster and Parsons (1938), and as a stain for 

 fungi in roots by Garrett (1937). 



S55 PHENOL RED 



pK = 7.9 



This is phenol-sulfonphthaleih: 



HO OH 



O 



-I 

 SO 



2 



An indicator changing from yellow to red, employed for some time 

 in physiological work, and well known before the paper by Clark 

 and Lubs above mentioned. The range of this indicator (approx. 

 pH 6.8-8.4) makes it quite valuable in much biological work; it is 

 quite commonly used in the study of kidney function. 



S60 CRESOL RED 



pK = 8.3 



This is ortho-cresol-sulfonphthalein : 



HO OH 



\/\ 



CH 



An indicator changing from yellow to red 



S65 CRESOLPHTHALEIN 



pK = 9.4 



This indicator is closely related to phenolphthalein, having the 

 constitution : 



HO OH 



CH 



