CHAPTER 9 



The Classification of Dyes 



Dyes are classified by their chromophores. In a logical classifica- 

 tion the natural dyes such as haematein and carmine do not form a 

 group apart, but divide themselves here and there among the 

 various groups of dyes, interspersed among the synthetic ones. 

 Haematein, for instance, owes its colour to a paraquinonoid ring, 

 and is therefore much more closely related to the fuchsines than 

 to a dye such as orange G, which owes its colour to an entirely 

 diiTerent chromophore. 



Nearly all the dyes used in microtechnique ow^e their colour 

 either to a quinonoid ring, or to an azo linkage (-N=N-, p. 182), 

 or to a nitro-group (-NO 2, p. 184). There are thus three main 

 groups, of which the first is so diverse that it requires considerable 

 subdivision, while the third (nitro dyes) is so small that it can be 

 dealt with very shortly. 



THE QUINONOID DYES 



These mostly contain a paraquinonoid ring, but in some cases 

 an orthoquinonoid ring is present instead (p. 181). 



II 



Paraquinofioid rijig Orthoquinonoid ring 



The chief subdivisions of the quinonoid dyes are represented 

 here by skeleton formulae, from which the auxochromes and 

 modifiers are omitted. The chromophore is indicated in each case 

 in a particular resonance-position. 



The 'X' in the formula for the azines and related dyes may 

 represent nitrogen, oxygen, or sulphur. The double bonds of the 

 quinonoid ring of this group of dyes cannot be represented in full 



169 



