Sodium thiosulfate

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Sodium thiosulfate
Names
IUPAC name
Sodium thiosulfate
Systematic IUPAC name
Sodium thiosulfate
Other names
Sodium thiosulphate
Sodium hyposulfite
Hyposulphite of soda
Properties
Na2S2O3
Molar mass 158.11 g/mol (anhydrous)
248.18 g/mol (pentahydrate)
Appearance White crystalline solid
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 ethanol, methanol
Vapor pressure ~ 0 mmHg
Hazards
Safety data sheet MSDS
Flash point Non-flammable
Related compounds
Related compounds
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 heptahydrate Na2S2O3·5H2O. This solid is slightly efflorescent.

Properties

Chemical

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

Physical

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

Availability

Sodium thiosulfate is sometimes sold in pharmacies or pet stores.

Preparation

Sodium thiosulfate can be prepared by boiling an aqueous sodium hydroxide solution with elemental sulfur, but isolation of the pure solid 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.

Projects

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

Handling

Safety

Sodium thiosulfate may cause irritation on contact.

Storage

In closed containers.

Disposal

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

References

Relevant Sciencemadness threads