Sodium Thiosulfate
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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)
Na2S2O3 · 5H2O 248.18
Na2S2O3 158.11
Thiosulfuric acid, disodium salt, pentahydrate;
Disodium thiosulfate pentahydrate CAS RN®: 10102-17-7.
Anhydrous CAS RN®: 7772-98-7.
1 DEFINITION
Sodium Thiosulfate contains NLT 99.0% and NMT 100.5% of sodium thiosulfate (Na2S2O3), calculated on the anhydrous basis.
2 IDENTIFICATION
Change to read:
A. Spectroscopic Identification Tests 〈197〉, Infrared Spectroscopy: 197A (CN 1-May-2020)
B. Identification Tests—General, Sodium〈191〉
Sample solution: 0.1 g/mL in water
Acceptance criteria: Meets the requirements
3 ASSAY
Procedure
Sample: 800 mg
Titrimetric system
(See Titrimetry 〈541〉.)
Mode: Direct titration
Titrant: 0.1 N iodine VS
Endpoint detection: Visual
Analysis: Dissolve the Sample in 30 mL of water. Adjust with a suitable solution of dilute hydrochloric acid or dilute sodium hydroxide to a pH of 6.2–6.7, if necessary. Titrate with Titrant, adding 3 mL of starch TS as the endpoint is approached. Each mL of Titrant is equivalent to 15.81 mg of sodium thiosulfate (Na2S2O3).
Acceptance criteria: 99.0%–100.5% on the anhydrous basis
4 IMPURITIES
Calcium
Sample solution: Dissolve 1 g in 20 mL of water.
Analysis: To the Sample solution add a few mL of ammonium oxalate TS.
Acceptance criteria: No turbidity is produced.
5 SPECIFIC TESTS
Water Determination, Method III〈921〉
Sample: 1.0 g
Analysis: Dry the Sample under vacuum at 40°–45° for 16 h.
Acceptance criteria: 32.0%–37.0%
6 ADDITIONAL REQUIREMENTS
Packaging and Storage: Preserve in tight containers.
USP Reference Standards 〈11〉
USP Sodium Thiosulfate RS
