Not A Mordant

Mordants are metals.

However, not all metals are mordants. The commonest mordant metals in histotechnology are aluminum and ferric iron, both with valencies of three. Less commonly, other metals may be used as mordants, such as beryllium, calcium, chromium, lead, tungsten, molybdenum etc.. By definition, however, a mordant must be a metal. If a chemical is not metallic, it is not a mordant.

Note that iodine is not a metal. Therefore it is not a mordant. It's function in Gram's stain is as a trapping agent.

Similarly, the use of Bouin's fluid before Masson's trichrome is an example of secondary fixation of sections. Picric acid is not a metal and consequently is not a mordant. Bouin's fluid contains no metal ions, and therefore cannot mordant tissues. It can, and does, refix them to make them more avid for acid dyes. This is a well known characteristic of picric acid fixation, used to good effect here.



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