A chemical used in the technique shown for both oxidation and differentiation is which of the following?

• A. Ferric chloride
• B. Potassium permanganate
• C. Iodine
• D. Sodium thiosulfate

Answer: (A)

Ferric chloride provides ferric ions that both activate hematoxylin and help form the iron-hematoxylin complex used to stain nuclei. Hematoxylin must be oxidized to its active form (hematein) to bind tissue, and ferric ions in ferric chloride do this oxidation while also acting as the mordant that holds the dye in place. This iron-hematein complex yields a crisp nuclear stain, and the staining can be fine-tuned by differentiation steps that remove excess color, producing clear contrast between nuclei and surrounding tissue. Potassium permanganate is a stronger, less selective oxidizer; iodine and sodium thiosulfate have different roles (mordanting, decolorizing) that don’t fit this specific oxidation-plus-differentiation context.