Potassium 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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1 DEFINITION

Potassium Metabisulfite contains an amount of K₂S₂O₅ equivalent to NLT 51.8% and NMT 57.6% of SO₂.

2 IDENTIFICATION

A. Identification Tests—General, Potassium 〈191〉 and Sulfite 〈191〉:

A solution (1 in 20) meets the requirements.

3 ASSAY

Procedure

Sample: 250 mg of Potassium Metabisulfite

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: Transfer the Sample 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 min, protected from light, and then add 1 mL of hydrochloric acid. Titrate the excess iodine with 0.1 N sodium thiosulfate VS, adding 3 mL of starch TS as the endpoint is approached.

Calculate the percentage of SO₂ in the portion of Sample taken:

Result = {[(Vᵦ − Vₛ) × N × F] / W} × 100

Vᵦ = Back titrant volume consumed by the Blank (mL)

Vₛ = Back titrant volume consumed by the Sample (mL)

N = Back titrant actual normality (mEq/mL)

F = equivalency factor, 32.03 mg/mEq

W = weight of the Sample (mg)

Acceptance criteria: 51.8%–57.6% of SO₂

4 IMPURITIES

Change to read:

▲Iron 〈241〉, Procedures, Procedure 1▲ (CN 1-Jun-2023)

Test preparation: Dissolve 1.00 g 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.

Acceptance criteria: NMT 10 ppm

5 ADDITIONAL REQUIREMENTS

Packaging and Storage:

Preserve in well-fitted, tight containers, and avoid exposure to excessive heat.