Which chemical removes excess Schiff reagent during the staining process?

• A. Metabisulfite
• B. Sodium thiosulfate
• C. Hydrogen peroxide
• D. Acetic acid

Answer: (A)

In the Periodic Acid–Schiff (PAS) staining process, removing excess Schiff reagent is done by a reducing agent that differentiates the stain. Metabisulfite acts this role, converting any unbound Schiff reagent back to a colorless form. This selective reduction prevents background color and ensures that only tissues with aldehyde groups (generated by periodic acid oxidation) retain the magenta color. That’s why metabisulfite is the correct choice—it specifically removes excess Schiff reagent and sharpens stain specificity. The other reagents don’t provide this differentiation step: hydrogen peroxide would bleach or alter color without selective reduction, acetic acid changes pH but doesn’t remove Schiff reagent, and sodium thiosulfate serves different purposes in other staining contexts.