THE CHEMICAL COMPOSITION OF DYES 



91 



Beyond auxochromes and chromophores, dyes often possess 

 atoms or groups of atoms that are called modifiers. Thus rosani- 



NH 



NH, 



+ 

 Rosaniline 



line differs from pararosaniline only in the possession of a single 

 methyl group; this modifies the colour, making it very slightly 

 bluer. (Basic fuchsine is a mixture of these two closely related 

 dyes.) The methyl group is here a modifier. It has been mentioned 

 that the hydrogens of basic auxochromes may also be replaced 

 by methyl or ethyl or aryl groups ; this again modifies the colour. 

 Each replacement of one of these hydrogens in pararosaniline by 

 methyl makes a bluer dye, and cr)^stal violet, in which all six are 

 replaced, is nearly blue. Ethyl and particularly aryl groups have 

 even more blueing effect, and purely blue dyes such as methyl 

 blue (p. 94) may be obtained in this way. 



Dyes are classified into groups by their chromophores. Most 

 of these groups contain both cationic and anionic dyes, possessing 

 the auxochromes mentioned above ; many of them in all groups 

 have modifiers (frequently methyl or ethyl). The great majority 

 of the dyes used in microtechnique owe their colour either to the 

 quinonoid ring or to the azo-group, -N=N- (p. 96). The 

 quinonoid dyes, however, are so diverse that it is necessary to 

 divide them into sub-groups, each characterized by a particular 

 chromophore that can be represented in a skeleton formula. The 

 principles of dyeing in microtechnique can be explained by the 

 use of only a few dyes, belonging to a small number of groups. 



