Sulfur Dioxide
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This article is compiled based on the United States Pharmacopeia (USP) – 2025 Edition
Issued and maintained by the United States Pharmacopeial Convention (USP)
SO₂ 64.06
Sulfur dioxide CAS RN®: 7446-09-5.
1 DEFINITION
Sulfur Dioxide contains NLT 97.0%, by volume, of sulfur dioxide (SO₂).
[Caution-Sulfur Dioxide is poisonous.]
2 ASSAY
2.1 Procedure
Sample: 100.0 mL of gaseous Sulfur Dioxide
Titrimetric system
- (See Titrimetry 〈541〉.)
- Mode: Direct titration
- Titrant: 0.1 N iodine VS
- Endpoint detection: Visual
Analysis: Collect the Sample over mercury, and note the temperature of the Sample and the pressure upon it. Slowly introduce 50.0 mL of 0.1 N sodium hydroxide into the air space over the mercury, and absorb the Sample in the solution by shaking. When absorption is complete, transfer the solution to a 250-mL conical flask, add 3 mL of starch TS, and titrate with Titrant until the solution is pale blue in color.
Calculate the percentage of sulfur dioxide (SO₂) at a temperature of 0° and a pressure of 760 mm of mercury in the portion of Sulfur Dioxide taken. Each mL of 0.1 N iodine is equivalent to 1.094 mL of SO₂.
Acceptance criteria: NLT 97.0%, by volume
3 IMPURITIES
3.1 Limit of Nonvolatile Residue
Sample: 300 g (209 mL)
Analysis: Transfer the Sample to a tared, 250-mL conical flask, and allow the liquid to evaporate spontaneously in a well-ventilated hood. When evaporation appears complete, blow a current of dry, filtered air through the flask until the odor of sulfur dioxide is no longer apparent.
Acceptance criteria: NMT 7.5 mg (0.0025%)
3.2 Sulfuric Acid
Analysis: To the flask containing the residue obtained in the test for Limit of Nonvolatile Residue add 25 mL of water previously neutralized to methyl red TS. Swirl the flask, and titrate with 0.10 N sodium hydroxide.
Acceptance criteria: NMT 1.3 mL is required (about 0.002%).
4 SPECIFIC TESTS
4.1 Water Determination, Method I 〈921〉
Sample: 3 g (2.1 mL)
Analysis: Taking precautions to avoid absorption of moisture, transfer the Sample to a suitable flask, and add 20 mL of anhydrous pyridine.
Acceptance criteria: NMT 2.0%
5 ADDITIONAL REQUIREMENTS
Packaging and Storage: Preserve in cylinders.
[Note-Sulfur Dioxide is used most in the form of a gas in pharmaceutical applications, and is described herein for such purposes. However, it is usually packaged under pressure, hence the preceding specifications are designed for testing it in liquid form.]
