General Nature of Dyes 27 



OH 



O2N I NO2 



NO2 



is an acid, capable of electrolytic dissociation and of forming salts 

 with alkalies. It is the familiar substance picric acid, and is a 

 yellow dye. 



It will thus be seen that the color of picric acid is due to the 

 chromophoric nitro groups, and that its dyeing properties are due 

 to the auxochromic hydroxyl group. If the nitro groups be re- 

 duced to amino groups (-NH2), which are not chromophores, the 

 resulting compound is colorless and hence is not a dye. 



Summing up, we arrive at the definition of a dye as an organic 

 compound which contains chromophoric and auxochromic groups 

 attached to benzene rings, the color being attributable to the chro- 

 mophores and the dyeing property to the salt-forming auxochromes. 



ACID AND BASIC DYES 



There is frequent misunderstanding among biologists as to the 

 meaning of the terms "acid dyes" and "basic dyes". It is some- 

 times assumed that the terms refer to the H-ion concentration of 

 the dye solutions. Nothing could be further from the truth. The 

 distinction actually depends on whether the significant part of the 

 dye is anionic or cationic, and bears no direct relation to the reac- 

 tion of any solution of the dye. As a matter of fact, some of the 

 most strongly anionic or acid dyes, like eosin Y, regularly form 

 salts with strong metallic ions such as sodium; and the greater 

 strength of the metallic cation determines the reaction of the solu- 

 tion, i.e. the solution is ordinarily basic in reaction. 



As the auxochromes are the salt forming groups of dyes, it is 

 ordinarily the nature of the auxochrome present that determines 

 in which of these two classes a dye belongs. 



Some auxochromes are basic, e.g., the amino group (-NH2), 

 while others are acidic, e.g., the carbonyl group (-COOH). The 

 amino group owes its basic character (which it transmits to the 

 whole molecule) to the ability of its nitrogen atom to become pen- 

 tavalent by the addition of the elements of water (or of an acid), 

 just as in the case of ammonia; thus: 



H H H H H. H H 



