General Nature of Dyes 29 



the word "base". Thus "basic fuchsin" means the salt of fuchsin 

 with some colorless acid (e.g., hydrochloric) while "fuchsin base" 

 indicates the color base of this dye, not combined with any acid. 



THE CHROMOPHORES 



As stated above, every dye contains at least one group of atoms 

 known as a chromophore, which is regarded as being responsible 

 for the colored properties of the compounds in which it occurs. 

 Some of these chromophores have a basic character, others acid. 

 There are only a comparatively small number of them which enter 

 into the usual biological stains, and only these need be considered 

 here. They are as follows : 



BASIC CHROMOPHORES 



1. The azo group, — N = N — , which is found in all azo dyes, of 

 which methyl oraiige and Bismarck brown are well known ex- 

 amples. In all these dyes, a benzene ring is attached to each 

 nitrogen atom. All the dyes of this group may be looked upon as 

 derivatives of azobenzene, 



N = N 



2. The azin groujp^ 



N 

 / 



\ 



N 



/ 



which is found in phenazins, of which neutral red and the safranins 

 are good representatives. The skeleton formula of a safranin is: 



H2N N NH 



in which x represents the negative ion of a monobasic acid such as 

 hydrochloric, acetic, nitric or sulfuric. This chromophore is 

 capable of a variety of rearrangements of its valency bonds, as the 

 bond between the two nitrogen atoms may disappear and the com- 



