Na2S2O3·5H2O 248.19

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

Anhydrous. 158.11 [7772-98-7].

Packaging and storage— Preserve in tight containers.

Identification—

Water á921ñ— Dry about 1.0g,accurately weighed,in vacuum at 40to 45for 16hours:it loses between 32.0%and 37.0%of its weight.

Calcium— Dissolve 1g in 20mLof water,and add a few mLof ammonium oxalate TS:no turbidity is produced.

Heavy metals á231ñ— Dissolve 1g in 10mLof water,slowly add 5mLof 3Nhydrochloric acid,evaporate on a steam bath nearly to dryness,and heat the residue at 150for 1hour.Add 15mLof water to the residue,boil gently for 2minutes,and filter.Heat the filtrate to boiling,and add sufficient bromine TSto produce a clear solution and provide a slight excess of bromine.Boil the solution to expel the excess bromine,cool to room temperature,add 1drop of phenolphthalein TS,and neutralize with 1Nsodium hydroxide.Dilute with water to 25mL:the limit is 0.002%.

Assay— Dissolve about 800mg of Sodium Thiosulfate,accurately weighed,in 30mLof water.If necessary,adjust by the addition of 3Nhydrochloric acid to a pHof between 6.2and 6.7and titrate with 0.1Niodine VS,adding 3mLof starch TSas the endpoint is approached.Each mLof 0.1Niodine is equivalent to 15.81mg of Na2S2O3.

Expert Committee:(PA4)Pharmaceutical Analysis 4

USP28–NF23Page 1795

Phone Number:1-301-816-8251