Acid dye

[1] Some acid dyes are used as food colorants,[2][3] and some can also be used to stain organelles in the medical field.

Compared to most acids, water has a much higher pKa value, meaning that it dissociates to give H+ with more difficulty.

In contrast, basic dyes are used to stain cell nuclei and some other acidic components of tissues.

Some examples include erythrosine, tartrazine, sunset yellow and allura red, to name a few, many of which are azo dyes.

In order to prevent health hazards, a dye must be approved for consumption before it can be marked as edible.

This includes their wet fastness, migration ability, and dyeing pH:[1] Some dyes are mutagenic and carcinogenic, including methyl orange, acid red 26, and trypan blue.