Which statement about ferric chloride in staining is correct?

• A. It is both a mordant and an oxidizer.
• B. It is an inert solvent for dyes.
• C. It is only used as a dehydrating agent.
• D. It stains cell membranes red.

Answer: (A)

Ferric chloride serves two roles in iron-hematoxylin staining: it acts as a mordant and as an oxidizing agent. As a mordant, the ferric ion binds with hematoxylin to form a complex that attaches strongly to tissue, especially nuclei, producing the characteristic blue-black nuclear stain. As an oxidizer, ferric chloride converts hematoxylin to hematein, enabling the dye–metal complex to form and affix to the tissue more effectively. This combination of mordant action and oxidation is what makes ferric chloride essential in this staining method. It is not an inert solvent, nor a dehydrating agent, and it does not stain membranes red—the latter is typically achieved by counterstains such as eosin.