The concentration of ethanol used to remove excess picric acid after Bouin's is:

• A. 70%
• B. 95%
• C. 50%
• D. 10%

Answer: (A)

After Bouin’s fixation, the tissue contains residual picric acid, which can give a yellow stain and interfere with subsequent staining. To remove it, you use an ethanol bath because picric acid is soluble in alcohol–water mixtures. A 70% ethanol solution is ideal because the water present helps dissolve the residual picric acid while beginning dehydration without being too harsh on the tissue. Higher concentrations like 95% ethanol dehydrate more aggressively and are less effective at removing picric acid, while lower concentrations such as 50% or 10% don’t dissolve it well enough. So, 70% ethanol efficiently clears excess picric acid and prepares the tissue for dehydration and embedding.