Sodium bisulfite (or sodium bisulphite, 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 odour of sulfur dioxide. Sodium bisulfite is used in the Modified Gomori Methenamine-Silver Stain Kit.