After fixing tissue in Bouin's solution, the excess picric acid is removed by washing in which solution?

• A. 70% Ethanol
• B. 95% Ethanol
• C. Water
• D. 70% Methanol

Answer: (A)

Bouin’s fixative contains picric acid, which can leave a yellow stain and interfere with staining if not removed. Washing with 70% ethanol is used because ethanol dissolves and carries away residual picric acid, while the 70% water content provides a gentle balance that begins dehydration without overly harshly shrinking or damaging tissue. This step helps ensure a clean slide for subsequent processing. Water alone does not effectively remove picric acid, and very high concentrations like 95% ethanol are more dehydrating and can introduce artifacts or excessive shrinkage. Methanol is not routinely used here due to greater toxicity and different effects on tissue.