Dyes are generally defined along the lines of being coloured, aromatic compounds that can ionise. Although this definition infers that ionic interaction with oppositely charged tissue constituents is the norm, there are exceptions. Some dyes require the presence of a metal to properly develop their colour or staining selectivity. These are termed mordant dyes.
The Colour Index uses this as a classification and naming system.
Each dye is named according to the pattern:–
mordant + base colour + number
These dyes are thereby specifically identified as dyes of the stated colour, and whose primary staining mechanism requires the presence of metal atoms. 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 most commonly used mordant dyes have hydroxyl and carboxyl groups and are negatively charged, i.e. anionic. It is convenient to view these as a specialised subgroup of acid dyes. Some other mordant dyes may possess amino groups, and are cationic overall. Despite this, they must still have hydroxyl or carboxyl groups, since lake formation requires it. Mordant dyes can usually stain by ionic interaction in the same way as other ionisable dyes. The colour is often pale, sometimes so pale that the results have no value.
It is often noted that when a mordant dye forms a lake with a metal, there is a strong colour change. This is because metals have low energy atoms. The incorporation of these low energy atoms into the delocalised electron system of the dye causes a bathochromic shift in the absorption. It is this delocalised electron system which is fundamentally responsible for colour in dyes. Since different metal atoms have differing energy levels, the colour of the lakes may also differ.
The most commonly used mordant dye is undoubtedly hematein (natural black 1), whose status as a natural product supercedes its mode of dyeing, apparently. Others are chromoxane cyanine R (mordant blue 3) and celestine blue B (mordant blue 14), both used as substitutes for alum hematoxylin but with a ferric salt as the mordant. Alizarin red S (mordant red 3) is valuable for the demonstration of calcium, particularly in embryo skeletons.
There are numerous other examples.