Dyes are generally defined along the lines of being coloured, aromatic compounds that can ionise. They are thus able to interact with oppositely charged tissue constituents. Dyes which are negatively charged, and are used to bind to positively charged tissue components, are termed acid dyes.
The Colour Index uses this as a classification and naming system.
Each dye is named according to the pattern:–
These dyes are thereby specifically identified as acid dyes of the stated colour, and whose primary mechanism of staining is by ionic bonding. Note that this is a functional and colour classification. It contains no chemical information, neither does it imply that dyes with similar names but unique numbers are in any way related. It should also be noted that the classification refers to the primary mechanism of staining. Other mechanisms may also be possible.
The groups that are responsible for the ionising capability are the auxochromes. Acid dyes have hydroxyl, carboxyl or sulphonic groups as their auxochromes, and consequently have an overall negative charge. In other words, the coloured part of the molecule is the anion. Although the molecular charge is often shown on a specific atom in structural formulas, it is the whole molecule that is charged.
An example of a dye with hydroxyl groups as the auxochrome is martius yellow (acid yellow 24).
The carboxyl group is also negatively charged, but is usually found in conjunction with other groups rather than as the sole auxochrome. This is illustrated in the dye eosin Y (acid red 87) which has a hydroxyl group as well.
The final negatively charged auxochrome is the sulphonic group. This is also found in conjunction with other auxochromes. An example is biebrich scarlet (acid red 66), which also has a hydroxyl group. The sulphonic group frequently has the effect of causing an otherwise positively charged dye to have an overall negative charge, as demonstrated by acid fuchsin (acid violet 19), which is sulphonated pararosanilin (basic red 9).
There are numerous other examples.