Sodium Metabisulfite

This article is compiled based on the United States Pharmacopeia (USP) – 2025 Edition

Issued and maintained by the United States Pharmacopeial Convention (USP)

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Na₂S₂O₅ 190.11

Disulfurous acid, disodium salt;

Disodium pyrosulfite CAS RN^®: 7681-57-4.

1 DEFINITION

Sodium Metabisulfite contains an amount of sodium metabisulfite (Na₂S₂O₅) equivalent to NLT 65.0% and NMT 67.4% of SO₂.

2 IDENTIFICATION

A. IDENTIFICATION TESTS-GENERAL, Sodium (191)andSulfite (191); A solution (1 in 20) meets the requirements.

3 ASSAY

PROCEDURE

Sample: 200 mg of Sodium Metabisulfite

Blank: 50.0 mL of 0.1 N iodine VS, accurately measured

Titrimetric system

(See Titrimetry (541).)

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 sulfur dioxide (SO₂) in the portion of Sodium Metabisulfite taken:

Result = {[(V_B - V_S) x N x F/W} x 100

V_B = Back-titrant volume consumed by the Blank (mL)

V_S = 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

4 IMPURITIES

4.1 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 Visual Comparison (630)).

Acceptance criteria: Any turbidity produced by the Sample solution does not exceed that of the Standard solution (0.05%).

4.2 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%).

Change to read:

4.3 IRON (241), Procedures, Procedure 1 (CN, June 1, 2023)

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

5 ADDITIONAL REQUIREMENTS

PACKAGING AND STORAGE: Preserve in well-filled, tight containers, and avoid exposure to excessive heat.