Which reagent is commonly used to remove formalin pigment from tissue sections?

• A. Alcoholic picric acid
• B. Sodium hypochlorite
• C. Sodium thiosulfate
• D. Hydrogen peroxide

Answer: (A)

Formalin pigment, also called acid hematin, forms in tissues fixed with formalin and exposed to air; it appears as brownish to black pigment that can obscure cellular detail. Alcoholic picric acid is the standard reagent to remove this pigment because the pigment dissolves in the alcohol-picric acid medium, forming a soluble complex that can be washed away without aggressively disturbing the tissue or the hematoxylin stain. The other reagents aren’t used for this purpose: sodium hypochlorite is a strong bleach that can damage tissue and stains, hydrogen peroxide bleaches pigments and can alter staining, and sodium thiosulfate is typically used for other clearing or decolorizing tasks, not for formalin pigment removal.