Sodium Metabisulfite
(soe' dee um met'' a bye sul' fite).

Na2S2O5 190.11
Disulfurous acid, disodium salt;
Disodium pyrosulfite     [7681-57-4].
DEFINITION
Sodium Metabisulfite contains an amount of sodium metabisulfite (Na2S2O5) equivalent to NLT 65.0% and NMT 67.4% of SO2.
IDENTIFICATION
A solution (1 in 20) meets the requirements.
ASSAY
•  Procedure
Sample:  200 mg of Sodium Metabisulfite
Blank:  50.0 mL of 0.1 N iodine VS, accurately measured
Titrimetric system
Mode:  Residual titration
Titrant:  0.1 N iodine VS
Back-titrant:  0.1 N sodium thiosulfate VS
Endpoint detection:  Visual
Analysis:  Add the Sample to 50.0 mL of 0.1 N iodine VS in a glass-stoppered conical flask, and swirl to dissolve. Allow to stand for 5 min, protected from light. Add 1 mL of hydrochloric acid, and titrate the excess iodine with Back-titrant, adding 3 mL of starch TS as the endpoint is approached. Perform a blank determination.
Calculate the percentage of sodium metabisulfite (Na2S2O5) in the portion of Sodium Metabisulfite taken:
Result = {[(VB VS) × N × F]/W} × 100
 VB = = Back-titrant volume consumed by the Blank (mL) VS = = Back-titrant volume consumed by the Sample (mL) N = = Back-titrant normality (mEq/mL) F = = equivalency factor, 32.03 mg/mEq W = = Sample weight (mg)
Acceptance criteria:  65.0%–67.4% of SO2
IMPURITIES
Test preparation:  1 g
Analysis:  Dissolve the Test preparation in 10 mL water. Add 5 mL of hydrochloric acid, evaporate on a steam bath to dryness, and dissolve the residue in 25 mL of water.
Acceptance criteria:  NMT 20 ppm
•  Limit of Chloride
Standard solution:  0.71 mL of 0.020 N hydrochloric acid in 100 mL of water
Sample solution:  1.0 g in 10 mL of water. [Note—Pass through a small chloride-free filter, if necessary. ] Add 6 mL of 30% hydrogen peroxide. Add 1 N sodium hydroxide until the solution is slightly alkaline to phenolphthalein, and dilute with water to 100 mL.
Analysis
Samples:  Standard solution and Sample solution
Dilute 2.0 mL of the Samples with water to 20 mL. Add 1 mL of nitric acid and 1 mL of silver nitrate TS. Allow to stand for 5 min protected from direct sunlight, and compare the turbidity from the Samples (see Spectrophotometry and Light-Scattering 851, Visual Comparison).
Acceptance criteria:  Any turbidity produced by the Sample solution does not exceed that of the Standard solution (0.05%).
•  Limit of Thiosulfate
Standard solution:  Mix 0.10 mL of 0.10 N sodium thiosulfate with 10 mL of 1 N hydrochloric acid.
Sample solution:  Mix 2.2 g with 10 mL of 1 N hydrochloric acid.
Analysis
Samples:  Standard solution and Sample solution
Gently boil the Samples for 5 min. Cool, then transfer each solution to a small test tube.
Acceptance criteria:  Any turbidity produced by the Sample solution does not exceed that of the Standard solution (0.05%).
Test preparation:  Dissolve 500 mg of Sodium Metabisulfite 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, then dilute with water to 47 mL.
Analysis:  Proceed as directed in the chapter.
Acceptance criteria:  NMT 20 ppm