Sodium Formaldehyde Sulfoxylate

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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CH₃NaO₃S 118.09

CH₃NaO₃S · 2H₂O 154.11

Methanesulfinic acid, hydroxy-, monosodium salt;

Monosodium hydroxymethanesulfinate CAS RN®: 149-44-0.

Dihydrate CAS RN®: 6035-47-8.

1 DEFINITION

Sodium Formaldehyde Sulfoxylate contains an amount of sodium formaldehyde sulfoxylate (CH₃NaO₃S) equivalent to NLT 45.5% and NMT 54.5% of SO₂, calculated on the dried basis. It may contain a suitable stabilizer, such as sodium carbonate.

2 IDENTIFICATION

2.1 A.

Sample solution: Dissolve 4 g in 10 mL of water in a test tube.

Analysis: To the Sample solution add 1 mL of silver–ammonia–nitrate TS.

Acceptance criteria: Metallic silver is produced, either as a finely divided, gray precipitate or as a bright metallic mirror on the inner surface of the tube.

2.2 B.

Sample solution: Dissolve 40 mg of salicylic acid in 5 mL of sulfuric acid, and add 50 mg of Sodium Formaldehyde Sulfoxylate.

Analysis: Warm very gently.

Acceptance criteria: A permanent, deep red color appears.

3 ASSAY

3.1 Procedure

Sample: 1 g

Titrimetric system

• (See Titrimetry 〈541〉.)
• Mode: Direct titration
• Titrant: 0.1 N iodine VS. [Note-Prepare an adequate amount for both the Assay and the test for Sodium Sulfite.]
• Endpoint detection: Visual

Analysis: Transfer the Sample to a 50-mL volumetric flask, dissolve in 25 mL of water, and dilute with water to volume. Reserve a portion of this solution for the test for Sodium Sulfite. Transfer 4.0 mL of the remaining solution to a conical flask containing 100 mL of water. Titrate with Titrant, adding 3 mL of starch TS as the endpoint is approached. Each mL of 0.1 N iodine is equivalent to 1.602 mg of SO₂.

Acceptance criteria: 45.5%–54.5% of SO₂ on the dried basis

4 IMPURITIES

4.1 Sulfide

Analysis: Dissolve 6 g in 14 mL of water in a test tube, and wet a strip of lead acetate test paper with the clear solution.

Acceptance criteria: No discoloration is evident within 5 min.

4.2 Iron

Standard solution: Dissolve 43.2 mg of ferric ammonium sulfate in 10 mL of 2 N sulfuric acid, and add water to make 1000 mL, each mL representing 5 µg of Fe.

Sample solution: Transfer 1.0 g of Sodium Formaldehyde Sulfoxylate to a suitable crucible, and carefully ignite, initially at a low temperature until thoroughly charred, and finally, preferably in a muffle furnace, at 500°–600° until the carbon is all burned off. Cool, dissolve the residue in 2 mL of hydrochloric acid, and dilute with water to 50 mL.

Analysis: To 5.0 mL of the Standard solution and 50 mL of the Sample solution add 50 mg of ammonium persulfate and 5 mL of ammonium thiocyanate TS, and transfer each to a separate color comparison tube.

Acceptance criteria: 0.0025%; the color of the Sample is not deeper than that of the Standard solution.

4.3 Sodium Sulfite

Sample solution: 4.0 mL of the solution prepared for the Assay in a conical flask containing 100 mL of water

Titrimetric system

• (See Titrimetry 〈541〉.)
• Mode: Direct titration
• Titrant: 0.1 N iodine VS, prepared in the Assay
• Endpoint detection: Visual

Analysis: Add 2 mL of formaldehyde TS to the Sample solution, and titrate with the Titrant, adding 3 mL of starch TS as the endpoint is approached.

Calculate the percentage of sodium sulfite (Na₂SO₃) in the Sodium Formaldehyde Sulfoxylate taken:

Result = (V₂ − V₁) × (N/W) × (F × 1.25)

V₂ = volume of 0.1 N iodine VS consumed in the titration performed in the Assay (mL)

V₁ = volume of 0.1 N iodine VS consumed in this titration (mL)

N = actual normality of the Titrant (mEq/mL)

W = weight of the Sample in the Assay (g)

F = equivalency weight of sodium sulfite, 63.02 mg/mEq

Acceptance criteria: NMT 5.0% on the dried basis

5 SPECIFIC TESTS

5.1 pH 〈791〉

Sample solution: 20 mg/mL

Acceptance criteria: 9.5–10.5

5.2 Loss on Drying 〈731〉

Analysis: Dry at 105° for 3 h.

Acceptance criteria: NMT 27.0%

5.3 Alkalinity

Sample solution: 1.0 g of Sodium Formaldehyde Sulfoxylate in 50 mL of water

Analysis: To the Sample solution add phenolphthalein TS, and titrate with 0.10 N sulfuric acid.

Acceptance criteria: NMT 3.5 mL is required for neutralization.

5.4 Clarity and Color of Solution

Sample solution: 1 g of Sodium Formaldehyde Sulfoxylate in 20 mL of water

Analysis: Transfer 10 mL of the Sample solution to a 20- × 150-mm test tube. Compare with water in a similar test tube.

Acceptance criteria: The Sample solution and the water are equally clear and, when viewed transversely by transmitted light, exhibit no apparent difference in color.

6 ADDITIONAL REQUIREMENTS

Packaging and Storage: Preserve in well-closed, light-resistant containers, and store at controlled room temperature.