CHAPTER IV 



DYES OF THE NITRO, AZO, AND OXYQUINONE GROUPS 



1. THE NITRO GROUP 



In this group the chromophore is -NO2. The chromophore is of 

 such a strongly acid character that the dyes of this group are all 

 acid dyes. The best known nitro dye is picric acid. 



PICRIC ACID c. I. NO. 7* 



Picric acid is formed by the action of nitric acid on phenol, 

 thus introducing three nitro groups : 



OH 



O.N— r >— NO. 



NO. 



(An acid dye; absorption maximum about 360 in alcohol) 

 This compound forms salts by the dissociation of the-OH group, 

 and the salts have considerable value as stains. Ammonium 

 picrate is the one most commonly thus used. 



Picric acid (or one of its salts) is quite extensively employed in 

 contrast to acid fuchsin in the VanGieson connective tissue stain. 

 It is also used as a general cytoplasmic stain in contrast to the 

 basic dyes. It has further application as a fixative for tissues 

 that are to be sectioned.! 



MARTIUS YELLOW C. I. NO. 9 



Synonyms: Manchester yellow, Naphthol yellow. 



OH 



—NO. 



NO. 



(An acid dye; absorption maxima about J^}^5, \39d, Sld\) 

 Martins yellow has been used by Pianese in combination with 

 malachite green and acid fuchsin for studying cancer tissue; the 

 same technic was applied to plant tissue by Midler, and is now quite 



*This abbreviation stands for the number in the "Colour Index"; see Chapter 

 II, p. 23. 



fFor bibliographic references concerning the procedures referred to in this 

 chapter see Table 2 in Appendix I, pp. 110-128, and also the bibliography in 

 Appendix III, p. 138. 



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