Nitro and Azo Dyes 69 



It is obsolete as a textile dye, although sometimes used as a photo- 

 graphic desensitizer. It is called for in combination with toluidine 

 blue and acid fuchsin in the Champy-Kull technic for demonstrat- 

 ing certain cell constituents (mitochondria, etc.) It is very 

 poisonous, frequently causing severe dermatitis. 



Kassanis (1939) employs it in the KuU technic (i.e. with acid 

 fuchsin and toluidine blue) for the study of virus infested plants. 

 Favorsky (1939) uses it for inducing polyploidy in plants. 



3. THE AZO GROUP 



The azo dyes are characterized by the chromophore — N = N — 

 joining benzene or naphthalene rings, thus: 



N = N 



It is possible for the azo group to occur more than once in a mole- 

 cule, forming the disazo dyes, thus: 



N=N 



kA , 



N=N 



The azo chrombphore is distinctly basic; but not sufficiently so to 

 make the dyes basic when they contain hydroxyl groups. Those 

 containing amino groups are, of course, pronouncedly basic. 



The position of the hydroxyl or amino group on a benzene ring 

 in relation to the azo group is important. Ordinarily they are in 

 the para position to each other, thus : 



(a 



OH 



The ortho position is next frequently assumed; rarely the meta 

 position. When the hydroxyl group assumes the ortho position the 

 character of the compound is quite distinct from that of the para 

 compounds. By a rearrangement of the atoms such a compound 

 is sure to change to a quinoid form, thus : 



H O 



A compound of this latter structure cannot form salts and does not 

 act as an ordinary dye. It does, however, prove to be soluble in 



