Mercuric oxide was used in the original Harris hematoxylin formula to form which compound?

• A. Hematein
• B. Hematoxylin
• C. Hematin
• D. Mordant complex

Answer: (A)

Mercuric oxide acts as the oxidizing agent that converts hematoxylin into hematein. Hematein is the active form that can bind to tissue in the presence of a mordant, forming the stable hematoxylin–mordant complex responsible for the nuclear stain. In the Harris formula, oxidation by mercuric oxide produces hematein, which then pairs with the mordant to give the characteristic stain. Hematoxylin is the starting dye before oxidation; hematatin is a different compound and not the product here; the mordant complex refers to the final bound form, not the immediate product of oxidation.