206 



Biological Stains 



It is a newer indicator than phenolphthalein, and is generally 

 regarded as preferable when used in conjunction with the sulfon- 

 phthaleins, because its range does not overlap quite so far the 

 alkaline range of thymol blue. It is a one-color indicator and its 

 color change is the same as that of phenolphthalein. 



S70 PHENOLPHTHALEIN 



pK = 9.7 



This indicator, of which the formula is given on p. 198, is quite 

 useful in the alkaline range centering around pH 9.7, with a color 

 change from red in alkaline solutions to colorless in acid ones. It 

 has the disadvantage of a one-color indicator discussed on p. 200. 



5. ACRIDINE DYES 



There is a small group of dyes derived from the compound 

 acridine ^x ^ 



1 

 H 



./ 



which is closely related to xanthene. They are not xanthene dyes, 

 but are included in this chapter for convenience. There are a few 

 of interest to the biologist. 



To 



ACRIDINE YELLOW 



H 



I 

 H.2N_/\_N_/\_,NH2 



CHa 



CI 



CH. 



C. I. NO. 785 



-c=^ 



I 



H 



C15H16N3CI; Mol. Wt. 273.759 

 [A basic dye; absorption maximum about 455 (Formdnek)] 



This dye is rarely called for in microtechnic, but has been re- 

 ferred to by Metcalf and Patton (1944) as a fluorochrome useful in 

 insect histology. 



