CHAPTER X 



MISCELLANEOUS DYES, PIGMENTS, AND HISTO- 



CHEMICAL REAGENTS 



1. THE ANTHRAQUINONE GROUP 

 The anthraquinone dyes include derivatives of anthracene, 



224 Biological Stains 



through its oxidation product anthraquinone: 



O 



These dyes show the quinoid ring, which is the most important 

 chromophore in nearly all the dyes to be discussed in the three fol- 

 lowing chapters; it forms very strong chromogens, which require 

 only the addition of auxochrome groups to be converted into strong 

 dyes, either basic or acid. The chromogen anthraquinone is con- 

 verted into a dye by the addition of hydroxyl groups, its best 

 known. derivatives among the dyes being: 1:2 dihydroxy-anthra- 

 quinone (alizarin) and 1:2:4 trihydroxy-anthraquinone (purpurin). 

 Both of these compounds occur in nature in the root of madder, 

 being the colored principles of madder extract. They have the 

 property of combining with metallic oxides to form so-called 

 "lakes," insoluble compounds of different color from the dye enter- 

 ing into them. This makes them valuable ones to use after mor- 

 danting with aluminium, iron or chromium compounds. 



El 



ALIZARIN C. I. NO. IO2.7 



(Various grades denoted as: Alizarin P, VI, le.) 



C14H8O4; Mol. Wt. 240.204 

 (An axdd dye; absorption maxima about \610.8], 566.5^ \527.6\ 



in alkaline solution) 



Solubility at 26°C: in water nil; in alcohol 0.125% 



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