SECTION ONE 



** dye " is probably absorbed or dissolved by lipids to which 

 colour is imparted. A few non-ionic " dyes " such as Sudan black, 

 which has two basic groups (imino), might conceivably unite 

 chemically with acidic elements of certain tissues. All " neutral 

 dyes " can be converted into true dyes by the process of sulphon- 

 ation. They then become water-soluble dyes. In the strict 

 sense a dye is a coloured aromatic, organic substance capable 

 of electrolytic dissociation which enables it to unite by chemical 

 and/or physical means with tissue-elements or other substances 

 of opposite charge. 



Neutral " dyes " should not be confused with " neutral " or 

 compound stains. The latter are formed by chemical union 

 between a basic and an acid dye. Such compounds of basic and 

 acid dyes have long been known to histologists. The histologists 

 were the first to invent them. It is doubtful, however, if the 

 average textile dyestuff manufacturer is aware of such preparations. 

 It is still more doubtful if they would be of any interest to the 

 textile dyer. 



COMPOUND DYES 



As stated above these are neutral dyes of the kind that appear 

 to be known only to the histologist. They are produced by 

 neutralizing suitable acid dyes with suitable basic dyes. That 

 is to say, the negatively charged dye-ion of the acid dye unites 

 with the positively charged dye-ion of the basic dye when aque- 

 ous solutions of the two dyes are brought together, to form a 

 precipitate which is a two-colour dye. If, however, one of the 

 dyes is a polychrome stain, then the resultant compound stain 

 will also possess the same polychrome properties plus the 

 colour of the other dye. Such a compound dye is a true chemical 

 compound and not a mixture. Among such dyes are Jenner stain 

 (Jenner, 1899), Leishman stain (Leishman, 1901), methyl green- 

 orange G (Kardos, 1911), neutral red-light green (Twort, 1924), 

 and neutral red-fast green FCF (Ollet, 1947, 1951)- 



As will be seen from the following example which shows the re- 

 action involved in the union of an acid dye (eosin) with a basic dye 

 (methylene blue), an exchange of ions takes place. The sodium 

 from the eosin unites with chloride from the methylene blue, 

 and the organic part of one dye which is positively charged unites 



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