Sodium thiosulfate

Lore & Background

Sodium thiosulfate is prepared by oxidation of sodium sulfite with sulfur or by boiling aqueous sodium hydroxide and sulfur. It decomposes upon heating to 300 °C into sodium sulfate and sodium polysulfide, and acidification leads to complete decomposition into sulfur, sulfur dioxide, and water. The thiosulfate anion is tetrahedral in the solid state, with a sulfur-sulfur single bond.

Reader's Guide

Sodium thiosulfate's significance stems from its dual role as a reducing agent and ligand. In medicine, it is on the World Health Organization's List of Essential Medicines for treating cyanide poisoning and was approved by the U.S. FDA in 2022 under the trade name Pedmark to reduce hearing loss in pediatric cancer patients receiving cisplatin. In photography, it was a key fixer in the 19th century, dissolving silver halides. It is also used to dechlorinate water, in iodometry as a titrant, and in organic chemistry to produce Bunte salts. Its low toxicity (LDLo for rabbits 4000 mg/kg) supports its widespread use.

Did You Know?

• Sodium thiosulfate is sometimes still called 'hypo' from its original chemical name, hyposulphite of soda.
• It is used to treat cyanide poisoning and is on the World Health Organization's List of Essential Medicines.
• In September 2022, the U.S. FDA approved sodium thiosulfate under the trade name Pedmark to lessen hearing loss in cancer patients receiving cisplatin.
• It reacts with bromine to remove free bromine from solution and is used for safe disposal of bromine and iodine.