What chemical acts as a mordant in hematoxylin solutions?

• A. Ammonium aluminum sulfate
• B. Sodium iodate
• C. Ferric chloride
• D. Methyl green

Answer: (A)

In hematoxylin staining, a mordant is a substance that forms a stable complex with the oxidized form of the dye (hematein) and tissue, so the stain attaches firmly to structures like nuclei. The aluminum salt provided by ammonium aluminum sulfate supplies aluminum ions that bind with hematein to form a hematein–aluminum complex. This complex has a strong affinity for nucleic acids in the nucleus, producing the characteristic blue-purple nuclear stain. Sodium iodate is used to oxidize hematoxylin to hematein, not to act as the mordant. Ferric chloride can be used as a mordant in some iron-based hematoxylin variants, but in standard aluminum-hematein hematoxylin solutions, the mordant is the aluminum salt. Methyl green is simply another dye used as a counterstain, not a mordant.