Molecular formula. Na₂S₂O₃,5H₂O

Relative molecular mass. 248.2

Chemical name. Disodium thiosulfate pentahydrate; disodium thiosulfate (Na₂S₂O₃) pentahydrate; thiosulfuric acid (H₂S₂O₃), disodium salt, pentahydrate; CAS Reg. No. 10102-17-7 (pentahydrate).

Description. Transparent, colourless crystals; odourless.

Solubility. Very soluble in water; practically insoluble in ethanol (~750 g/l) TS.

Category. Antidote.

Storage. Sodium thiosulfate should be kept in a tightly closed container.

Additional information. Sodium thiosulfate effloresces in dry air at temperatures exceeding 33°C and dissolves in its water of crystallization at about 49°C.

Requirements

Definition. Sodium thiosulfate contains not less than 99.0% and not more than 101.0% of Na₂S₂O₃,5H₂O.

Identity tests

A. Dissolve 1.0 g in 10 mL of water (use this solution also for tests B and C). To 2 mL add 1.0 mL of iodine TS; the colour of the solution is discharged. Add 0.25 mL of barium chloride (50 g/l) TS; the solution remains clear.

B. To 2 mL of the solution prepared in test A add 1.0 mL of hydrochloric acid (~70 g/l) TS; an odour of sulfur dioxide is perceptible and a precipitate of sulfur is produced (proceed with caution).

C. To 2 mL of the solution prepared in test A add 2 mL of silver nitrate (0.1 mol/l) VS; a white precipitate is formed, but quickly becomes yellowish, then black.

D. When tested for sodium as described under 2.1 General identification tests yields the characteristic reactions. If reaction B is to be used, prepare a 0.10 g/mL solution.

Heavy metals. Dissolve 1.0 g in 10 mL of water, add slowly 5 mL of hydrochloric acid (~70 g/l) TS, and evaporate to dryness on a water-bath. Add 15 mL of water to the residue, boil gently for 2 minutes and filter. Heat the filtrate to boiling, add sufficient bromine TS1 to the hot filtrate to produce a clear solution and provide a slight excess of bromine. Boil the solution to expel the excess bromine. Cool, add 0.05 mL of phenolphthalein/ethanol TS and neutralize with sodium hydroxide (1 mol/l) VS. Adjust the pH to 3-4 with acetic acid (~60 g/l) TS, dilute to 40 mL with water, and determine the heavy metals content as described under 2.2.3 Limit test for heavy metals, according to Method A; not more than 20 μg/g.

Chlorides. Dissolve 1.20 g in a mixture of 20 mL of water and 2 mL of nitric acid (~130 g/l) TS, boil gently for 3-4 minutes, cool, filter, and proceed with the filtrate as described under 2.2.1 Limit test for chlorides; the chloride content is not more than 0.2 mg/g.

Sulfates and sulfites. Dissolve 0.25 g in 10 mL of water, add 5 mL of iodine TS, and gradually more iodine TS, drop by drop, until a very faint persistent yellow colour is produced. Proceed as described under 2.2.2 Limit test for sulfates; the sulfate content is not more than 2 mg/g.

Sulfides. Dissolve 1 g in 10 mL of water and add 0.05 mL of sodium nitroprusside (45 g/l) TS; the solution does not become violet.

Clarity and colour. A solution of 1.0 g in 10 mL of carbon-dioxide-free water R is clear and colourless.

pH value. pH of a 0.10 g/mL solution in carbon-dioxide-free water R, 6.0-8.4.

Assay. Dissolve about 0.5 g, accurately weighed, in 25 mL of water and titrate with iodine (0.05 mol/l) VS using starch TS as indicator, added towards the end of the titration. Each mL of iodine (0.05 mol/l) VS is equivalent to 24.82 mg of Na₂S₂O₃,5H₂O.

Additional requirements for Sodium thiosulfate for parenteral use

Complies with the monograph for "Parenteral preparations".

Bacterial endotoxins. Carry out the test as described under 3.4 Test for bacterial endotoxins; contains not more than 0.03 IU of endotoxin RS per mg.