Difference between Sodium Metabisulfite and Sodium Bisulfite?
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 |
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.
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