Hematein is formed in Mayer hematoxylin solution by the addition of which chemical?

• A. Sodium iodate
• B. Sodium thiosulfate
• C. Potassium dichromate
• D. Ammonium chloride

Answer: (A)

Hematoxylin must be oxidized to hematein to become the active staining form that binds to tissue via a mordant. In Mayer hematoxylin solution, sodium iodate is the oxidizing agent that converts hematoxylin into hematein. Once formed, hematein binds with the aluminum mordant in the solution to create the stable blue‑purple nuclear stain. Sodium thiosulfate would bleach or reduce stains, potassium dichromate is a different oxidizer not used here, and ammonium chloride mainly adjusts the solution’s acidity without forming hematein.