Which additive counteracts the shrinking of cells in Bouin's fixative?

• A. Acetic acid
• B. Formic acid
• C. Ammonia
• D. Sodium bicarbonate

Answer: (A)

Acetic acid is the additive in Bouin's fixative that counteracts tissue shrinkage. Bouin's solution combines picric acid, formaldehyde, and acetic acid; the acetic acid component causes tissue swelling, balancing the strong fixation effects of the other components and helping preserve overall tissue dimensions and morphology. The other substances don’t provide this swelling effect in Bouin’s solution; formic acid isn’t used for this purpose, ammonia would alter the fixative’s pH in undesired ways, and sodium bicarbonate would neutralize the acidity, undermining fixation.