Formalin pigment may be removed from tissue by which solution?

• A. Alcoholic picric acid
• B. Acetic acid
• C. Sodium hydroxide
• D. Water

Answer: (A)

Formalin pigment is a brown to black pigment that can form in tissues fixed with formalin and exposed to air. It is an iron-containing hematin-like pigment that can interfere with staining. Alcoholic picric acid is used because it bleaches and dissolves this pigment, making it removable without severely damaging the tissue or the subsequent stains. Water or mild acids like acetic acid won’t effectively remove it, and strong bases such as sodium hydroxide can damage tissue components.