SECTION ONE 



ANTHRAQUINONE GROUP 



About twenty-five dyes of this group are used as microscopical 

 stains. Some of these (e.g. aHzarin red) are used as hydrogen 

 ion indicators also. All anthraquinone dyes may be regarded as 

 derivatives of anthraquinone, whose structure represents their 

 chromogen : 



ANTHRAQUINONE 

 O 



In two or three cases one of the oxygen atoms is replaced by 

 a nitrogen atom which links the chromogen to other aromatic 

 organic groups. 



To biologists perhaps the best known anthraquinone dye is 

 alizarin and its water-soluble derivative, alizarin red S (alizarin 

 sulphonate sodium). Alizarin was originally obtained from natural 

 sources; i.e. from madder root. 



Sudan violet, which is really a neutral " dye ", of the anthra- 

 quinone group, has two basic groups (amino). This dye is used 

 for staining lipids. It is completely insoluble in water, although 

 soluble to the extent of about i% in 30% ethyl alcohol. This dye 

 has the lowest molecular weight (210) of all organic dyes. 



Of the anthraquinone dyes used in biology, anthraquinone 

 violet has the highest molecular weight (633). 



Among the anthraquinone colours is carminic acid. It is the 

 active principle of carmine. Carminic acid is extracted from the 

 dried bodies of the female species. Coccus cacti, a tropical insect 

 which lives on certain species of cactus plants. At one time it 

 was in frequent use as an acid-base indicator, but it is now little 

 used for that purpose. Carmine, which is not a definite chemical 

 compound, is a crimson pigment of great historical interest. It 

 was used in micro-technique as far back as the eighteenth century ; 



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