Sodium bisulfite (or sodium bisulfite, sodium hydrogen sulfite) is a chemical mixture with the approximate chemical formula NaHSO₃. Sodium bisulfite in fact is not a real compound, but a mixture of salts that dissolve in water to give solutions composed of sodium and bisulfite ions. It is a white solid with an odor of sulfur dioxide. Regardless of its ill-defined nature, “sodium bisulfite” is a food additive with E number E222.