Sodium Thiosulphate

Na2S2O3-5H2O -- 248.19
Sodium Thiosulfate USP
Contains not less than 99.0 percent and not more than 100.5 percent of Na2S2O3, calculated on the anhydrous basis.
Identification—
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 and for Thiosulfate.
Water— Dry about 1.0 g, accurately weighed, in vacuum at 40o to 45o for 16 hours: it loses between 32.0% and 37.0% of its weight.
Calcium— Dissolve 1 g Sodium Thiosulfate in 20 mL of water, and add a few mL of ammonium oxalate TS: no turbidity is produced.
Heavy metals—: the limit is 0.002%.

Na2S2O3-5H2O
Sodium Thiosulphate BP Ph Eur
DEFINITION
Content
99.0 per cent to 101.0 per cent of Na2S2O3-5H2O.
CHARACTERS
Appearance
Transparent, colourless crystals, efflorescent in dry air.
Solubility
Very soluble in water, practically insoluble in ethanol 96 per cent. It dissolves in its water of crystallization at about 49C.
IDENTIFICATION
A. It decolorizes iodinated potassium iodide solution.
B. To 0.5 ml of solution S (see Tests) add 0.5 ml of water R and 2 ml of silver nitrate solution. A white precipitate is formed which rapidly becomes yellowish and then black.
C. To 2.5 ml of solution S add 2.5 ml of water and 1 ml of hydrochloric acid. A precipitate of sulphur is formed and gas is evolved which gives a blue colour to starch iodate paper.
D. 1 ml of solution S gives reaction of sodium.
TESTS
Solution S
Dissolve 10.0 g in carbon dioxide-free water R prepared from distilled water R and dilute to 100 ml with the same solvent.
Appearance of solution
The freshly prepared solution is clear and colourless.
Dissolve 10.0 g in 50 ml of distilled water, add 1 ml of 0.1 M sodium hydroxide and dilute to 100 ml with the same solvent.
pH
6.0 to 8.4 for the freshly prepared solution S.
Sulphates and sulphites
Maximum 0.2 per cent.
Sulphides
To 10 ml of solution S add 0.05 ml of a freshly prepared 50 g/l solution of sodium nitroprusside. The solution does not become violet.
Heavy metals
Maximum 10 ppm.
ASSAY
Dissolve Sodium Thiosulfate 0.500 g in 20 ml of water and titrate with 0.05 M iodine, using 1 ml of starch solution, added towards the end of the titration, as indicator.
1 ml of 0.05 M iodine is equivalent to 24.82 mg of Na2S2O3,5H2O.

Sodium Hyposulfite
Na2S2O3-5H2O Formula wt 248.19
DESCRIPTION
Sodium Thiosulfate or Sodium Thiosulphate occurs as large, colorless crystals or as a coarse, crystalline powder. It is deliquescent in moist air and effloresces in dry air at a temperature above 33°. Its solutions are neutral or faintly alkaline to litmus. One gram dissolves in 0.5 mL of water. It is insoluble in alcohol.
Function: Sequestrant; antioxidant.
REQUIREMENTS
Identification
A. Add a few drops of iodine TS to a 1:10 aqueous solution. The color disappears.
B. A 1:20 aqueous solution gives positive tests for Sodium and for Thiosulfate.
Assay: Not less than 99.0% and not more than 100.5% of Na2S2O3 after drying.
Lead: Not more than 2 mg/kg.
Selenium: Not more than 0.003%.
Water: Between 32.0% and 37.0%.