H H H H H H H 



/ \ \ / / \ / 



R— N + O = R— N and R— N +H— CI = R— N 



. \ / / \ \ / \ 



H H H OH H H CI 



amine water hypothetical amine hydro- amine 



organic am- chloric hy dro- 



mon ium base acid gas chloride 



The hydroxy! group, on the other hand, is weakly acidic, as it can 

 furnish hydrogen ions by electrolytic dissociation. The more of 

 either one of these two 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 hydroxyl 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, altho incapable in itself of conferring 

 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 extensive elec- 

 trolytic dissociation . This group, however, is only very feebly auxo- 

 chromic. 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 introduction 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 what has been said above, it is not to be presumed that 

 the dyes of commerce are actually bases or acids. Generally the 

 basic dyes are sold as salts of a colorless acid, such as hydrochloric, 

 sulfuric, oxalic or acetic acid. Likewise the acid dyes are sold as 

 their sodium, potassium, calcium or ammonium salts. Occasion- 

 ally the basic dyes are sold as the free bases, as for example the oil 

 soluble dyes (see p. 38). When a basic dye which is ordinarily sold 

 in the form of a salt comes into commerce as the free base, it is 

 customary to use the word "base" immediately after the name of 

 the dye. Thus, "basic fuschin" indicates a salt of fuchsin with a 

 colorless acid, while "fuchsin, base" indicates fuchsin itself, not 

 combined with an acid. 



THE CHROMOPHORES 



As stated above, every dye contains at least one group of atoms 

 known as a chromophore, which is regarded as being responsible 

 for the colored properties of the compounds in which it occurs. 

 Some of these chromophores have a basic character, others acid. 

 There are only a comparatively small number of them which enter 



16 



