Which role does sodium iodate play in hematoxylin staining preparations?

• A. Oxidizes hematoxylin to hematein
• B. Mordant
• C. Stains blue
• D. Fixative

Answer: (A)

Sodium iodate functions as an oxidizing agent that converts hematoxylin into hematein, the active form that binds to tissue. Once hematoxylin is oxidized to hematein, it forms a complex with the aluminum mordant (alum) in the stain, producing the characteristic blue nuclear stain. Without this oxidation step, hematoxylin would not become the staining species that attaches to nuclei as effectively. So the key role is to oxidize hematoxylin to hematein, enabling the blue hematoxylin–hematein–mordant complex to form. This agent is not the mordant itself, nor a fixative, and it does not stain blue by itself—the blue color comes from the hematein–mordant complex.