Sodium Thiosulfate
Na2S2O3·5H2O 248.19

Thiosulfuric acid, disodium salt, pentahydrate.
Disodium thiosulfate pentahydrate. [10102-17-7].

Anhydrous. 158.11 [7772-98-7].
» Sodium Thiosulfate contains not less than 99.0 percent and not more than 100.5 percent of Na2S2O3, calculated on the anhydrous basis.
Packaging and storage— Preserve in tight containers.
A: To a solution (1 in 10) add a few drops of iodine TS: the color is discharged.
B: A solution (1 in 10) responds to the tests for Sodium 191 and for Thiosulfate 191.
Water 921 Dry about 1.0 g, accurately weighed, in vacuum at 40 to 45 for 16 hours: it loses between 32.0% and 37.0% of its weight.
Calcium— Dissolve 1 g in 20 mL of water, and add a few mL of ammonium oxalate TS: no turbidity is produced.
Heavy metals 231 Dissolve 1 g in 10 mL of water, slowly add 5 mL of 3 N hydrochloric acid, evaporate on a steam bath nearly to dryness, and heat the residue at 150 for 1 hour. Add 15 mL of water to the residue, boil gently for 2 minutes, and filter. Heat the filtrate to boiling, and add sufficient bromine TS to produce a clear solution and provide a slight excess of bromine. Boil the solution to expel the excess bromine, cool to room temperature, add 1 drop of phenolphthalein TS, and neutralize with 1 N sodium hydroxide. Dilute with water to 25 mL: the limit is 0.002%.
Assay— Dissolve about 800 mg of Sodium Thiosulfate, accurately weighed, in 30 mL of water. If necessary, adjust by the addition of 3 N hydrochloric acid to a pH of between 6.2 and 6.7 and titrate with 0.1 N iodine VS, adding 3 mL of starch TS as the endpoint is approached. Each mL of 0.1 N iodine is equivalent to 15.81 mg of Na2S2O3.
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Monograph Elena Gonikberg, Ph.D.
Senior Scientist
(MDGRE05) Monograph Development-Gastrointestinal Renal and Endocrine
USP32–NF27 Page 3586
Pharmacopeial Forum: Volume No. 29(5) Page 1579