Difference between Sodium Metabisulfite and Sodium Bisulfite? SMBS is commonly used for removal of free chlorine and as a biostatic. Other chemical reducing agents exist (e.g., sulfur dioxide), but they are not as cost-effective as SMBS. When dissolved in water, sodium bisulfite (SBS) is formed from SMBS:
Na₂S₂O₅ (sodium metabisulfite) + H₂O –> 2 NaHSO₃ (sodium bisulfite)
then reduces hypochlorous acid according to:
2NaHSO₃ + 2HOCl –> H₂SO₄ + 2HCl + Na₂SO₄
In theory, 1.34 mg of sodium metabisulfite will remove 1.0 mg of free chlorine. In practice, however, 3.0 mg of sodium metabisulfite is normally used to remove 1.0 mg of chlorine. The SMBS should be of food-grade quality and free of impurities. SMBS should not be cobalt-activated. Solid sodium metabisulfite has a typical shelf life of 4 to 6 months under cool, dry storage conditions. In aqueous solutions, however, sodium bisulfite can oxidize readily when exposed to air. A typical solution life can vary with concentration as follows:

Concentration (wt %)	Solution life
10	1 week
20	1 month
30	6 months

A SBS solution is made by dissolving solid sodium metabisulfite into water and has a pH of 4.6 at 1.0 % (by weight) solution strength. A 10 % (by weight) SBS solution will require the addition of 0.51 pounds of solid sodium metabisulfite into one gallon of water.
The sodium metabisulfite is commercially available at 97.5 to 99 % purity and can be stored safely up to six months in a dry storage area.