172 Biological Stains 



basic character. The compounds of this group are not very im- 

 portant as dyes and are scarcely used as stains. The greatest 

 interest of the biologist in them is due to their use as indicators. 

 In acid solution rosolic acid is yellow but it is converted by alkali 

 to the disodium salt which is red, thus: 



0=/~\=C 



pararosolic acid 



0= 



disodium pararosolate 



There is considerable confusion in the nomenclature of these 

 dyes, as the names employed may be used in a strict chemical sense 

 or in a looser sense in practice. Chemically there are two rosolic 

 acids, which are related just as are rosanilin and pararosanilin. 

 Pararosolic acid differs from pararosanilin, only in having hydroxyl 

 groups in place of the amino groups: 



H2N_/— \ /~\_NH. HO^/— \ /— \ OH 



s /V 

 C 



"\NH. / \/-X OH 



H \_/ H 



leuco-pararosanilin leuco-pararosolic acid 



Rosolic acid, on the other hand, is a mono-methyl derivative, and 

 bears the same relation to rosanilin: 



H,N_/— \ /— \_NH. H0_/— \ /— \ OH 



\ 



CH3 / \/-\_NH, CH3 / \/-\_0H 



H \_/ H \_/ 



leuco-rosanilin leuco-rosolic acid 



Now the dye to which the name rosolic acic' " aurin is generally 

 given in practice is a mixture consisting of b< ih rosolic acid and 

 pararosolic acid together with other closely related compounds. 

 This dye is: 



n90 aurin or rosolic acid C. I. NO. 72.4 



The above names apply to the free acid. Yellow corallin is the 

 sodium salt. 



