Periodic acid is used in the PAS technique as what?

• A. A reducing agent
• B. An oxidizer
• C. A mordant
• D. A dye

Answer: (B)

Periodic acid in the PAS method acts as an oxidizing agent. It oxidizes vicinal diols in carbohydrates and related glycoconjugates to reactive aldehyde groups. These aldehydes then react with Schiff reagent to produce the characteristic magenta color. It is not a reducing agent, not a mordant, and not a dye itself—the chemical’s role is to create aldehydes that the Schiff reagent can color. This makes PAS particularly useful for staining glycogen, mucosubstances, basement membranes, and related structures.