184 Biological Stains 



3. FLUORAN DERIVATIVES 



Fluoran is not a dye, but is a very important compound in dye 



chemistry. It is a derivative of phthalic anhj'^dride, and contains 



a xanthene ring (five C atoms and one O atom) as well as a lactone 



ring (four C atoms and one O atom) besides three benzene rings; 



thus: 



O 



The fluoran dyes are derivatives of this by the introduction of 

 hydroxyl groups into two of the benzene rings at the para position 

 to the central carbon atom and the further introduction of halogen 

 atoms at various positions in all three benzene rings. 



It proves convenient here to class these compounds with the 

 xanthene dyes. They may, however, be equally well considered 

 tri-phenyl-methane dyes, as can be seen by a glance at the formula 

 of any of them; in fact they are generally so considered by the 

 chemists. To the biologist they stand in a distinctly different class 

 from the tri-phenyl-methanes; and for that reason are treated here 

 instead of in the preceding chapter. The dyes of this group are 

 also, and equally correctly, spoken of as the fluorescein derivatives, 

 fluorescein being, as will be seen on p. 186, a salt of di-hydroxy- 

 fluoran. Sometimes, moreover, the fluorescein dyes are referred 

 to as the eosins. This term to cover the whole group is hardly 

 justified, as it is better to call eosins only the dyes definitely so 

 named below. 



The fluorescein dyes are all similar in their action, but with 

 certain decided differences. In order for the reader to obtain a 

 real understanding of these stains and their behavior, a general 

 discussion of the whole group is necessary. 



The dyes in this group most important to the biologist are: 

 eosin Y, C. I. No. 768 (also called eosin yellowish or eosin water 

 soluble); ethyl eosin, C. I. No. 770 (also called eosin alcohol 

 soluble) ; eosin B, C. I. No. 771 (also called eosin bluish) ; erythro- 

 sin B, C. I. No. 773; phloxine, C. I. No. 774; rose bengal, C. I. 

 No. 779. It is not yet absolutely certain which of these dyes is 

 most suitable for any particular purpose; but some information 

 is available on the subject. The difference in behavior of the 

 different dyes of this group seems to depend upon two factors: 

 difference in color and difference in acidity. The relation between 

 these two factors and chemical structure was discussed by Conn 



