STAINING, PRACTICAL AND THEORETICAL 



in tissues is, of course, something very different from the re- 

 moval of a dye by an ordinary solvent. Probably the reason many 

 sulphonated acid dyes usually hold on to tissue elements despite the 

 application of solvents is that the dyes are either trapped within 

 the tissues in the manner suggested (page 5), or because they 

 are in firm chemical union with basic tissue elements. As already 

 stated, sulphonated dyes rely upon their strongly acidic sulphonic 

 groups for their acidity. Presumably such groups are not found 

 in tissues under normal circumstances. Therefore when these 

 strongly acid dyestuffs are in contact with tissue-elements there 

 is apparently a strong attraction between the sulphonic groups of 

 the dye and the basic groups (e.g. amino, imino) of the tissue 

 elements. Hence, it seems reasonable to suppose that chemical 

 union takes place between the sulphonated dye and basic tissue- 

 elements. Since proteins are ubiquitous in tissues, so must be 

 their basic groups (e.g. amino, imino). This may explain in part 

 why many sulphonated acid dyes stain tissues with such inten- 

 sity. 



Here, another question might be asked. That is, why do some 

 sulphonated dyes replace others with which the tissues have 

 previously been stained. The answer may be found in the relative 

 degrees of acidity of the competing ions. The one replaced may 

 not be so strongly acid as the dye which replaces it in tissues. 

 Orange G, sun yellow G, chlorantine fast green BLL, for instance, 

 replace a number of less strongly acid dyes, including acid fuchsin 

 and light green SF. 



Presumably the strongly basic tissue elements, with which the 

 less strongly acid sulphonated dyes were first united, cast oflF the 

 latter for the more strongly acid sulphonated dyes for which the 

 tissue elements have a greater affinity. 



Another instance of competition between dyes and tissue- 

 elements might be mentioned here. That is, the use of the acid 

 dye light green for differentiating the basic dye, safranin. This 

 is a case of competition between a strongly acid dye and a weakly 

 acid tissue-element for a basic dye. Light green is a sulphonated 

 dye. It appears unlikely that a carboxylated (acid) dye, devoid 

 of sulphonic groups, such as eosin could be used in simple 

 aqueous solution for differentiating safranin or any other strongly 

 basic dye, because eosin is too weak an acid to capture basic 

 dyes from the acidic tissue elements which depend upon their 



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