Sodium Sulfite

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

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

DOWNLOAD PDF HERE

Na₂SO₃ 126.04 CAS RN®: 7757-83-7.

1 DEFINITION

Sodium Sulfite contains NLT 95.0% and NMT 100.5% of sodium sulfite (Na₂SO₃).

2 IDENTIFICATION

A.Sample solution: 50 mg/mL of Sodium Sulfite. [Note—Reserve portions of the solution so obtained for use in Identification test B and in the test for Color and Clarity of Solution.]

Analysis: Add a drop of phenolphthalein TS.

Acceptance criteria: A pink color is produced.

B. Identification Tests—General 〈191〉, Sulfate

Analysis: To 5 mL of the solution from Identification test A add 0.5 mL of iodine TS.

Acceptance criteria: The solution is colorless and meets the requirements of test A

C. Identification Tests—General 〈191〉, Sodium: Meets the requirements of test A

3 ASSAY

Procedure

Sample: 250 mg

Titrimetric system

(See Titrimetry 〈541〉.)

Mode: Residual titration

Titrant: 0.1 N iodine VS

Back titrant: 0.1 N sodium thiosulfate VS

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

Endpoint detection: Colorimetric

Analysis: Add the Sample to a 500-mL beaker, add 50.0 mL of 0.1 N iodine VS, accurately measured, and shake to dissolve. Add 1 mL of starch TS, and titrate with 0.1 N sodium thiosulfate VS to a clear endpoint. Perform a blank determination, and make any necessary correction. Calculate the percentage of sodium sulfite (Na₂SO₃) in the Sample taken:

Result = [(B − V) × N × F × 100]/W

B = 0.1 N sodium thiosulfate VS volume consumed by the Blank

V = 0.1 N sodium thiosulfate VS volume consumed by the Sample

N = actual normality of the Back titrant (mEq/mL)

F = equivalency factor, 63.0 mg/mEq

W = weight of Sample (mg)

Acceptance criteria: 95.0%–100.5%

4 IMPURITIES

4.1 Limit of Iron

Standard solution: Immediately before use, dilute 1 volume of Standard Iron Solution, prepared as directed under Iron 〈241〉, Procedures,

Procedure 1, to 10 mL with water. [Note—This solution contains the equivalent of 1 μg/mL of iron.]

Sample solution: 10.0 g of Sodium Sulfite in 25 mL of water. Shake until mostly dissolved, and add 15 mL of hydrochloric acid. Heat to boiling. Cool, and dilute with water to 100.0 mL. Use a 10-mL portion.

Analysis: To the Standard solution and the Sample solution, separately add 2 mL of a citric acid solution (200 g/L), and then add 0.1 mL of thioglycolic acid. Make alkaline with stronger ammonia water, and dilute with water to 20 mL. Allow to stand for 5 min.

Acceptance criteria: Any pink color in the Sample solution is not more intense than that in the Standard solution (NMT 10 ppm).

4.2 Limit of Selenium

[Caution—Selenium is toxic; handle with care.]

Selenium standard solution: 100 μg/mL of selenium is prepared as follows. Dissolve 0.1 g of metallic selenium in 2 mL of nitric acid.

Evaporate to dryness, add 2 mL of water, and evaporate to dryness. Repeat the addition of water and the evaporation to dryness three more times. Dissolve the residue so obtained in 50 mL of diluted hydrochloric acid. Transfer to a 1000-mL volumetric flask, and dilute with diluted hydrochloric acid to volume.

Standard solution: To 1.0 g of Sodium Sulfite add 0.2 mL of Selenium standard solution and 10 mL of formaldehyde TS, and slowly add 2 mL of hydrochloric acid. Heat in a water bath for 20 min.

Sample solution: To 3.0 g of Sodium Sulfite add 10 mL of formaldehyde TS, and slowly add 2 mL of hydrochloric acid.

Analysis: Heat the Standard solution and the Sample solution in a water bath for 20 min.

Acceptance criteria: Any pink color in the Sample solution is not more intense than that in the Standard solution (NMT 10 ppm).

Change to read:

4.3 Limit of Thiosulfates

Sample solution: 20 mg/mL of Sodium Sulfite

Analysis: To 100 mL of the Sample solution, add 10 mL of formaldehyde TS and 10 mL of acetic acid. Allow to stand for 5 min. Add 0.5 mL of starch TS, and titrate with 0.1 N iodine VS. Perform a blank determination (see Titrimetry 〈541〉(CN 1-Aug-2024) ), and note the difference in volumes required.

Acceptance criteria: The difference in volumes is NMT 0.15 mL (NMT 0.1%).

4.4 Limit of Zinc

Zinc standard stock solution: A solution of 1 mL of acetic acid and the amount of zinc sulfate equivalent to 0.440 g of ZnSO · 7H O in 100.0 mL of water. [Note—This solution contains the equivalent of 1000 μg/mL of Zn.]

Zinc standard solution: 25 μg/mL of zinc from Zinc standard stock solution

Standard solutions: Transfer 1.0-, 2.0-, and 4.0-mL portions of Zinc standard solution to separate 100-mL volumetric flasks. Dilute the contents of each flask with water to volume, and mix to obtain solutions having known concentrations of 0.25, 0.5, and 1.0 μg/mL of zinc.

Sample stock solution: 100 mg/mL of Sodium Sulfite is prepared as follows. To 10.0 g of Sodium Sulfite add 25 mL of water. Shake untilfimostly dissolved, and slowly add 15 mL of hydrochloric acid. Heat to boiling. Cool, and dilute with water to 100.0 mL.

Sample solution: 20.0 mg/mL of Sodium Sulfite from the Sample stock solution

4.5 Instrumental conditions

(See Atomic Absorption Spectroscopy 〈852〉.)

Mode: Atomic absorption spectrophotometry

Analytical wavelength: Zinc emission line at 213.9 nm

Lamp: Zinc hollow-cathode

Flame: Air–acetylene

4.6 Analysis

Samples: Standard solutions and the Sample solution

Plot the absorbances of the Standard solutions versus concentration of zinc, in μg/mL, and draw the straight line best fitting the plotted points. From the graph so obtained, determine the concentration of zinc, in μg/mL, in the Sample solution.

Acceptance criteria: NMT 25 ppm

5 SPECIFIC TESTS

Color and Clarity of Solution

Analysis: Examine the solution prepared for Identification test A.

Acceptance criteria: The solution is clear and colorless.

6 ADDITIONAL REQUIREMENTS

Packaging and Storage: Preserve in tight containers. Store at room temperature.