Which of the following is NOT listed as an example of an antioxidant used in aqueous systems?

• A. Ascorbic acid
• B. Sodium bisulfite
• C. Metabisulfite
• D. Sulfur dioxide

Answer: (D)

Antioxidants used in aqueous systems are substances that actively scavenge oxidants in water, protecting sensitive components. Ascorbic acid is a classic water-soluble antioxidant that donates electrons to neutralize reactive species. Sodium bisulfite and metabisulfite release sulfite ions, which rapidly react with oxidants and help prevent oxidation in solution. Sulfur dioxide, while related to sulfites, is not typically employed as an antioxidant in aqueous pharmaceutical formulations due to its volatility and regulatory considerations; it’s more commonly used as a preservative in foods and beverages. Therefore, sulfur dioxide is not listed as an example of an antioxidant used in aqueous systems.