Which solution is used to adjust pH to 2.5 for Alcian blue staining?

• A. 3% acetic acid solution (pH 2.5)
• B. 0.9% saline
• C. 0.01 M phosphate buffer, pH 7.4
• D. Distilled water

Answer: (A)

Alcian blue staining is designed to bind to acidic mucins, which depend on an acidic environment for selective binding. Setting the staining solution to pH 2.5 creates conditions where sulfated glycosaminoglycans are negatively charged enough to attract the dye, while neutral mucins are not stained. A mild acid is used to achieve and maintain this specific pH, and a 3% acetic acid solution reliably brings and holds the mixture at pH 2.5 without overly harsh effects on tissue. The other options won’t deliver the required acidity: saline is near neutral and doesn’t acidify; distilled water has no buffering to fix pH at 2.5; and a phosphate buffer at pH 7.4 is clearly alkaline. So the acetic acid solution is the right choice for achieving pH 2.5 in Alcian blue staining.