Sodium metabisulfite, also known as pyrosulfite, is used as a preservative and antioxidant in pharmaceutical products. Metabisulfite is not stable in aqueous solutions and quickly converts to sulfite. The peak in the chromatogram originates from the sulfite. This process is however calibrated using metabisulfite standard solutions, which is why the results are specified as sodium metabisulfite. This Application Note describes the determination of metabisulfite as sulfite in an ointment. It is dissolved in an aqueous solution containing formaldehyde to protect the sulfite against oxidation.