28 



Biological Staijis 



In other words, the NH2-group yields hydroxyl ions and enables 

 the compound to ionize and to act as a cation in forming salts; 

 such a dye is a basic dye. 



The carboxyl group (-COOH), on the other hand, is acidic, as it 

 can furnish hydrogen ions by electolytic dissociation. Many of 

 the most important acid dyes contain this group; as they ordinarily 

 occur as sodium salts, the characteristic group in these dyes is 

 -COONa. Other acid dyes contain the rather weaker auxochrome 

 group -OH, and form salts in -ONa. 



The more of one of these groups in a compound, the stronger 

 base or acid it becomes. If there is one of each, the basic character 

 of the amino group predominates, but is weakened by the influence 

 of the acidic group. The strength of both groups is also influ- 

 enced by other groups or atoms in the compound; thus, for ex- 

 ample, the chromophore -NO2, although incapable in itself of con- 

 ferring acid properties to the compound, exerts an influence to make 

 any hydroxyl group in the compound more strongly acidic, in other 

 words to become more highly dissociated electrolytically. 



One other group of atoms encountered in dye chemistry needs 

 explanation, namely the sulfonic group, -SO3H. It is a salt- 

 forming group of strongly acidic character, in that it suffers ex- 

 tensive electrolytic dissociation. This group, however, is only 

 very feebly auxochromic. Its function is to render a dye soluble in 

 water, or to change an otherwise basic dye into an acidic one, as in 

 the case of the fuchsins, where the strongly basic "fuchsins" are 

 changed into the strongly acid "acid fuchsins" merely by the intro- 

 duction of sulfonic groups into the former. A compound which 

 contains a chromophore group and a sulfonic group is not a dye, 

 however, unless there is also present a true auxochrome group. 



From this it can be understood what is meant by calling dyes 

 either basic or acidic. It neither indicates reaction nor does it 

 mean that the dyes of commerce are actually bases or acids. They 

 are ordinarily salts. An acid dye is a salt of a color acid — usually 

 its sodium salt, but occasionally a salt of potassium, calcium or 

 ammonium. A basic dye is a salt of a color base — usually a 

 chloride, but sometimes a sulfate or acetate. (The oil-soluble 

 dyes present an exception to this statement; see p. 76.) The terms 

 anionic and cationic dyes would be more suitable, but they have 

 never come into general usage. Basic dyes are sometimes avail- 

 able as free bases, in which case the name is ordinarily followed by 



