After fixing in Bouin's fixative, excess picric acid is commonly removed by washing in which concentration range of alcohol?

• A. 50-70% alcohol
• B. 70-90% alcohol
• C. 90-100% alcohol
• D. Water

Answer: (A)

After Bouin's fixation, you want to remove the residual picric acid that’s still in and around the tissue. Picric acid dissolves best in an alcohol–water mixture, not in pure alcohol. The 50–70% alcohol range provides enough water to solubilize picric acid and allow it to diffuse out, while still letting the ethanol penetrate the tissue and carry away the fixative. Using higher concentrations of alcohol (90–100%) can hinder removal because picric acid becomes less soluble in nearly pure alcohol, and water alone isn’t as effective at dissolving it or at promoting controlled diffusion. So washing in dilute alcohol around 50–70% is the standard approach to leach out excess picric acid safely and effectively.