Sodium thiosulfate

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Sodium thiosulfate
Sodium thiosulfate sample watchglass.jpg
Sodium thiosulfate pentahydrate crystals
IUPAC name
Sodium thiosulfate
Systematic IUPAC name
Sodium thiosulfate
Other names
Sodium thiosulphate
Sodium hyposulfite
Hyposulphite of soda
Molar mass 158.11 g/mol (anhydrous)
248.18 g/mol (pentahydrate)
Appearance White crystalline solid
Odor Odorless
Density 1.667 g/cm3
Melting point 48.3 °C (118.9 °F; 321.4 K) (pentahydrate)
Boiling point 100 °C (pentahydrate) 320 °C (608 °F; 593 K) (decomposes)
70.1 g/100 mL (20 °C)
231 g/100 mL (100 °C)
Solubility Almost insoluble in acetonitrile, ethanol, methanol
Vapor pressure ~ 0 mmHg
Safety data sheet MSDS
Flash point Non-flammable
Related compounds
Related compounds
Ammonium thiosulfate
Sodium metabisulfite
Sodium sulfite
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Sodium thiosulfate is an inorganic compound with the chemical formula Na2S2O3, widely used in titrations and as cyanide poisoning antidote. The most common form of this salt is the pentahydrate Na2S2O3·5H2O. This solid is slightly efflorescent.



Sodium thiosulfate reacts with acids, releasing sulfur, and sulfur dioxide fumes:

Na2S2O3 + 2 HCl → 2 NaCl + S + SO2 + H2O

The hydrated form has a low melting point of around 50 °C, but in fact it decomposes and the salt dissolves in its own water of crystallization. When heated more strongly, the water is driven off. When the anhydrous compound is heated to 300 °C, it decomposes to sodium sulfate and sodium polysulfide:

4 Na2S2O3 → 3 Na2SO4 + Na2S5


Sodium thiosulfate pentahydrate is an odorless white crystalline solid, soluble in water, but almost insoluble in alcohols.


Sodium thiosulfate pentahydrate is sold by many swimming pool suppliers, as "chlorine neutralizer", in solid form.

Sodium thiosulfate is sometimes sold in pharmacies or pet stores. It can be bought cheaply online.


Sodium thiosulfate can be prepared by boiling an aqueous sodium hydroxide solution with elemental sulfur, but isolation of the pure solid compound is not easy. The other reaction product is sulfide and polysulfide.

Sodium sulfite can also be used instead of sodium hydroxide. In that case, the only reaction product is sodium thiosulfate.


  • Iodine clock reaction
  • Neutralize bleach
  • Remove iodine stains
  • Cyanide poisoning antidote
  • Gold extraction
  • Supercooling demonstration
  • Redox titrations
  • Bunte salts synthesis
  • Thioglycolic acid synthesis



Sodium thiosulfate may cause irritation on contact.


Sodium thiosulfate should be kept in closed containers.


No special precautions need to be taken. Bleach can be used to neutralize it.


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