DYEING AND OTHER PROCESSES OF COLOURING 307 



tensely blue substance, indigo. This possesses a chromophore and 

 also side-groups corresponding to auxochromes, through which, 

 presumably, attachment to the fibre is maintained. The inability 



O 



II 



— Cn 



:c-C\ 



o 



The chromophore of indigo ^^^ 



of indigo to ionize directly in water or aqueous solutions prevents 

 us from regarding it unequivocally as a dye, yet the resemblance is 

 close. The chromophore is the same as in the microtechnical dye, 

 indigo-carmine; but the latter, derived from indigo by sulphona- 

 tion, is a typical, water-soluble acid dye of the slowly- diffusing 

 kind (p. 236). 



The 'azoic' dyes of the textile industry bear a resemblance to 

 indigo, for here again the dye is synthesized as an insoluble sub- 

 stance in the fibre; but the reaction is not one of oxidation. These 

 dyes received their name because of their resemblance to azo dyes. 

 Indeed, they owe their colour to the same chromophore, but in- 

 solubility prevents their being used in the same way. The fibre 

 is impregnated with a naphthol ([^-naphthol, for instance), which 

 attaches itself through its -OH group, and usually, in modern 

 practice, through another group as well (often -CONH-, ortho to 

 the -OH).^^^ These attaching-groups resemble auxochromes, 

 though the substance is colourless. The fibre is then treated with 

 an aromatic diazo compound, and this snaps on to the naphthol by 

 a -N=N- link. The resulting coloured substance is essentially a 

 non-sulphonated azo dye. It can also be made in the absence of the 

 fibre; but it cannot be used in this form, because it is insoluble. 



A somewhat similar process is used in a histochemical technique 

 for the recognition of tyrosine. Here, however, there is no pre- 

 liminary impregnation of the tissue with a naphthol or related 

 compound; on the contrary, one relies on the presence in the 

 tissues of a phenol that will take the place of the naphthol used in 

 azoic dyeing. It is because tyrosine possesses a phenyl ring that it 

 can be histochemically demonstrated by the production of what 

 is essentially an azoic dye. The reagent used for this purpose is 

 tetra-azotized benzidine. In the presence of tyrosine this substance 

 links itself to the phenyl ring of the amino-acid,^^^ with the form- 



