128 



STAINING 



dyes, therefore, are salts which owe their coloured properties to 

 the cation, while the anion is that of an inorganic acid (or an 

 organic acid having no coloured properties). Acid dyes, on the 

 other hand, contain an acid radical such as —OH, — SO3H, or 

 • — COOH and form salts with strong bases like sodium and potas- 

 sium, the resulting dyes containing their chromophore group or 

 groups in the anion, not the cation. Basic dyes on the market 

 ordinarily occur as chlorides, occasionally sulphates or acetates. 

 Acid dyes are ordinarily sodium salts, although sometimes salts 

 of potassium or occasionally of some other metal. 



237. Colour Acids and Bases. Although the above-mentioned 

 salts are the normal forms in which dyes are available, most acid 

 dyes may be obtained in the form of the colour acids and some 

 basic dyes in the form of the colour bases. Thus the colour acid of 



Br Br Br Br 



NaO- 



eosm. 



Br 



-0- 



= 

 —Br 



-COONa 



-COOH 



The latter compound is coloured like its salt, as might be inferred 

 from the fact that it retains the quinonoid ring. It is almost 

 insoluble in water, however (nearly all colour acids being alcohol- 

 soluble, but not water-soluble), and has no dye properties except 

 in the presence of alkali when it is converted into one of its salts. 

 The colour bases of most basic dyes, however, are known in 

 theory only, all efforts to convert such dyes into their bases 

 destroying the quinonoid structure and resulting in the production 

 of so-called pseudo-bases or leuco-bases. Thus pararosanilin, one 

 of the primary constituents of basic fuchsin, yields its leuco-base 

 as follows : — 



+ N0C1 



— NH, 



I'urarosanilin carbinol (oolourless) 



