Metabisulfite as Sulfite in Pharmaceutical Ointments

Sodium metabisulfite or pyrosulfite is used as preservative and antioxidant in pharmaceutical products. In water metabisulfite is not stable and is rapidly converted to sulfite. This Application Note describes the determination of metabisulfite as sulfite in an ointment using the 940 Professional IC Vario from Metrohm. The ointment is dissolved in an aqueous solution containing formaldehyde to prevent sulfite from further oxidation.