What is the difference between Sodium Metabisulfite and Sodium Bisulfite and how are they used?

Answer ID 2261   |    Updated 03/13/2018 08:01 PM

Sodium metabisulfite (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 + H2O --> 2 NaHSO3

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

TMTrademark of The Dow Chemical Company ("Dow") or an affiliated company of Dow



Answer Feedback
Rate the helpfulness of this answer by selecting between 1 (low) and 5 (high) stars.