zation. This reaction is ordinarily very readily reversable and 

 makes such dyes useful indicators of acidity. It is discussed more 

 fully under acid fuchsin (p. 64) and phenolphthalein (p. 83). 



CLASSIFICATION OF DYES 



On the basis of the chromophore present the simple synthetic 

 dyes are classified into several groups. If each of these groups were 

 characterized by a single color or by a few closely related colors, 

 dye chemistry would be a comparatively simple proposition. As a 

 matter of fact a single chromophore may occur in dyes of practi- 

 cally all colors of the rainbow. It is ordinarily impossible to deter- 

 mine, a priori, from the chemical formula of a dye what particular 

 color the compound may have; but there is, nevertheless, a certain 

 general rule which correlates chemical formula with color. In any 

 group of compounds, the simpler ones are converted into the more 

 complex by substitution of radicals for hydrogen atoms. In the 

 dyes the substituents are generally methyl or ethyl groups, or 

 sometimes phenyl groups. Now the general rule is that the larger 

 the number of hydrogen atoms that have been replaced by these 

 groups the deeper the color. The tendency is for the color of the 

 simplest dyes in any group of homologous compounds to be yellow, 

 passing thru red to violets and then greens and blues, as the homo- 

 logs become higher thru the introduction of successively larger 

 numbers of methyl or other substituting groups. Thus the com- 

 pound pararosanilin, which is very frequently sold as basic 

 fuchsin, but should more properly be called basic rubin, is a tri- 

 phenyl methane, with an amino group attached to each benzene 

 ring, but without any methyl groups; thus: 



H 



\ 



CI 



Rosanilin, which is similar in composition, but contains one methyl 

 group attached to one of the benzene rings, 



is a red very similar to pararosanilin but with less of a yellowish 

 cast. Now another methyl group may be introduced into each of the 

 other two benzene rings, and each one successively deepens the 

 shade of red, so that thehighesthomologof the series, new fuchsin: 



20 



