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Sodium Metabisulfite
Na2S2O5 190.11

Disulfurous acid, disodium salt.
Disodium pyrosulfite [7681-57-4].
» Sodium Metabisulfite contains an amount of Na2S2O5 equivalent to not less than 65.0 percent and not more than 67.4 percent of SO2.
Packaging and storage— Preserve in well-filled, tight containers, and avoid exposure to excessive heat.
Identification— A solution (1 in 20) responds to the tests for Sodium 191 and for Sulfite 191.
Limit of chloride— Dissolve 1.0 g in 10 mL of water, filter, if necessary, through a small chloride-free filter, and add 6 mL of 30 percent hydrogen peroxide. Add 1 N sodium hydroxide until the solution is slightly alkaline to phenolphthalein, dilute with water to 100 mL, and mix. Dilute 2.0 mL of this solution with water to 20 mL, and add 1 mL of nitric acid and 1 mL of silver nitrate TS. Mix, allow to stand for 5 minutes protected from direct sunlight, and compare the turbidity, if any, with that produced from a 2-mL aliquot of a control (prepared by diluting 0.71 mL of 0.020 N hydrochloric acid to 100 mL) in an equal volume of total solution containing the quantities of reagents used in the test (see Visual Comparison in the section Procedure, under Spectrophotometry and Light-scattering 851): any turbidity produced by the test specimen does not exceed that of the control preparation (0.05%).
Iron 241 Dissolve 500 mg in 14 mL of dilute hydrochloric acid (2 in 7), and evaporate on a steam bath to dryness. Dissolve the residue in 7 mL of dilute hydrochloric acid (2 in 7), and again evaporate to dryness. Dissolve the resulting residue in a mixture of 2 mL of hydrochloric acid and 20 mL of water, add 3 drops of bromine TS, and boil to expel the bromine. Cool, and dilute with water to 47 mL: the limit is 0.002%.
Limit of thiosulfate— Mix 2.2 g with 10 mL of 1 N hydrochloric acid in a 50-mL beaker. Boil gently for 5 minutes, cool, and transfer to a small test tube. Any turbidity is not greater than that produced by 0.10 mL of 0.10 N sodium thiosulfate, similarly treated (0.05%).
Heavy metals, Method I 231 Dissolve 1 g in 10 mL of water, add 5 mL of hydrochloric acid, evaporate on a steam bath to dryness, and dissolve the residue in 25 mL of water: the limit is 0.002%.
Residual solvents 467: meets the requirements.
(Official January 1, 2007)
Assay— Transfer about 200 mg of Sodium Metabisulfite, accurately weighed, to a glass-stoppered conical flask containing 50.0 mL of 0.1 N iodine VS, and swirl to dissolve. Allow to stand for 5 minutes, protected from light, add 1 mL of hydrochloric acid, and titrate the excess iodine with 0.1 N sodium thiosulfate VS, adding 3 mL of starch TS as the endpoint is approached. Each mL of 0.1 N iodine is equivalent to 3.203 mg of SO2.
Auxiliary Information— Staff Liaison : Catherine Sheehan, B.Sc., Scientist
Expert Committee : (EM105) Excipient Monographs 1
USP29–NF24 Page 3425
Phone Number : 1-301-816-8262