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A. Synthetic Additives
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Standard and Specification > Synthetic Additives > Sodium Metabisulfite
333. Sodium Metabisulfite   - Sodium Pyrosulfite
Chemical Formula Na2S2O5 
Molecular Weight 190.11 
[Compositional Specifications of Sodium Metabisulfite]
Content Sodium Metabisulfite should contain not less than 95.0% of sodium metabisulfite (Na2S2O5). 
Description Sodium Metabisulfite is white crystallite or crystalline powder with odor of sulfur dioxide. 
Identification Sodium Metabisulfite shows reactions of bisulfite and sodium salts in Identification. 
Purity (1) Clarity of Solution : When 1 g of Sodium Metabisulfite is dissolved in 100 ml of water, the turbidity of the solution should be slightly turbid or better.

(2) pH : pH of an aqueous solution (1→10) should be 4.0~4.5.

(3) Thiosulfates : When 10% aqueous solution of Sodium Metabisulfite is acidified with sulfuric acid or hydrochloric acid, the solution should be transparent.

(4) Arsenic : 0.25 g of Sodium Metabisulfite is precisely weighed into a 150-ml beaker, where 10 ml of water is added to dissolved the solid. 10 ml of nitric acid and 5 ml of sulfuric acid are carefully added to this solution, which is then evaporated to 5 ml in a steam bath. The resulting concentrate is heated on a hot plate until a white smoke of sulfuric acid evolves. After cooling, 10 ml of water is used to wash down the splashed material on the beaker wall. It is then heated again on a hot plate until a white smoke of sulfuric acid evolves. This process is repeated again. Water is added to bring the total volume to 10 ml, Test Solution. When Test Solution is tested for arsenic, the content should not be more than 4ppm.

(5) Heavy Metals : 2 g of Sodium Metabisulfite is precisely weighed in 10 ml of water, which is evaporated to dryness in a steam bath. the residue is dissolved in water (total volume = 25 ml). When this solution is tested for heavy metals, the content should not be more than 10ppm.

(6) Iron : 0.5 g of Sodium Metabisulfite is precisely weighed, where 2 ml of hydrochloric acid is added. It is then evaporated to dryness in a steam bath. 2 ml of hydrochloric acid and 20 ml of water are added to dissolve the residues, where a few drops of bromine solution. The resulting solution is boiled to remove bromine. After cooling, water is added to bring the total volume to 25 ml, where 50 mg of ammonium persulfate and 5 ml of ammonium thiocyanate solution are added. The resulting color of the solution should not be deeper than that of a reference solution (not more than 50ppm). The reference solution is prepared by following the same procedure as Test Solution with 2.5 ml of iron standard solution instead of Test Solution.

(7) Selenium : 200 mg of Sodium Metabisulfite and 100 mg of magnesium oxide are dissolved in 20 ml of water, Test Solution. It is then analyzed with Cold Vapor Type in Atomic Absorption Spectrophotometry. The absorption of the Test Solution should not be higher than that of Standard Solution (not more than 30ppm). Standard Solution is prepared by diluting 3 ml of Selenium standard solution to 100 ml with water and following the same procedure as Test Solution. 
Assay Approximately 0.2 g of Sodium Metabisulfite is precisely weighed into a flask with a stopper filled with 50 ml of 0.1 N iodine solution. It is then dissolved. The stopper is placed and the flask is set-aside for 5 minutes. After 1 ml of hydrochloric acid is added, the excess iodine is titrated with 0.1 N sodium thiosulfate solution (indicator : starch solution).

                   0.1 N iodine solution 1 ml = 4.753 mg of Na2S2O5 
Permitted Use Level of Sodium Metabisulfite Usage as sulfur dioxide should not exceed the specifications given below.

1. Dried gourd (flesh of gourd is sliced and dried.) : 5.0 g/kg
2. Molasses and starch syrup : 0.3 g/kg
3. Yout : 0.4 g/kg
4. Fruit wine : 0.35 g/kg
5. Fruit juice that is five times diluted prior to use, concentrated fruit juice, and processed fruits and vegetables : 0.15 g/kg
6. Dried fruits : 2 g/kg
7. Konjac flour (Konjac jelly is made from Konjac root.) : 0.9 g/kg
8. Shrimp flesh : 0.1 g/kg
9. Sugar : 0.02 g/kg
10. Fermented vinegar : 0.17 g/kg
11. Dried potatoes(powder, particle and thin layer of dried potatoes or fresh potatoes, which is cut and then heated and dried) : 0.5g/kg
12. Other food items[sesame seeds, legumes, potatoes and pulses, fruits,
    vegetables, and their simply processed forms (skinned or cut) : 0.03 g/kg