Quinone-imine Dyes 103 



i5 -L, 6-DICHLOROINDOPHENOL 



CI 



I 



=N_/^\_ONa ^ 0^/^\^N_/~X_OH 



CI (blue) CI (red) 



which on reduction becomes the following colorless compound 



CI 



HO /~\ NH /~^\ OH 



It will be seen that the type of color change shown by this in- 

 dicator is very similar to that of litmus (see p. 217). Litmus is a 

 non-synthetic product of unknown chemical composition; it is not 

 believed to be related to the indophenols, in spite of its similar be- 

 havior as an acid-base and oxidation-reduction indicator. 



There is only one other dye of this group that need be mentioned 

 here. 



t20 indophenol blue c. I. NO .8ii 



Synonym: Indophenol. 



CII3 



0-. ~ _ _, . 



N 



CH3 



CisHieNzO; Mol. Wt. 276.324 



{Absorption maximum in alcohol about 592) 



This compound is occasionally referred to in biological work. 

 Herxheimer (1901), for example, employed a dye which he called 

 indophenol (and which probably was indophenol blue), in satu- 

 rated solution in 70% alcohol, as a fat stain. It is still sometimes 

 used for the same purpose; see Black (1938), for instance. 



3. THE THIAZINS 



The thiazins constitute one of the most important groups of 

 dyes from the standpoint of the biologist; while for textile dyeing 

 the group contains but a small number of dyes of any importance. 

 In these compounds, as mentioned above, the two benzene rings 

 are further joined by a sulfur atom. 



Considerable information is at hand concerning the spectro- 



