Sodium thiosulfate (sodium thiosulphate) is an inorganic compound with the formula . Typically it is available as the white or colorless pentahydrate (x = 5), which is a white solid that dissolves well in water. The compound is a reducing agent and a ligand, and these properties underpin its applications. Historically known as "hyposulphite of soda", sodium thiosulfate is used in medicine as a treatment for cyanide poisoning. It is also a reliable photographic fixer and played a key role in the development of photography in the 19th century.
Uses
Sodium thiosulfate is used predominantly in dyeing. It converts some dyes to their soluble colorless "leuco" forms. It is also used to bleach "wool, cotton, silk, soaps, glues, clay, sand, bauxite, and edible oils, edible fats, and gelatin." It is on the World Health Organization's List of Essential Medicines. Other uses include topical treatment of ringworm and tinea versicolor, and treating some side effects of hemodialysis and chemotherapy. In September 2022, the U.S. Food and Drug Administration (FDA) approved sodium thiosulfate under the trade name Pedmark to lessen the risk of ototoxicity and hearing loss in infant, child, and adolescent cancer patients receiving the chemotherapy medication cisplatin.
Photographic processing
In photography, sodium thiosulfate is used in both film and photographic paper processing as a photographic fixer. It is sometimes still called "hypo" from the original chemical name, hyposulphite of soda. It functions to dissolve silver halides, e.g., AgBr, components of photographic emulsions. Ammonium thiosulfate is typically preferred to sodium thiosulfate for this application.
This salt can also be prepared by boiling aqueous sodium hydroxide and sulfur according to the following equation. However, this is not recommended outside of a laboratory, as exposure to hydrogen sulfide can result if improperly handled.
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Principal reactions
Upon heating to 300 °C, it decomposes to sodium sulfate and sodium polysulfide:
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Thiosulfate salts characteristically decompose upon treatment with acids. Initial protonation occurs at sulfur. When the protonation is conducted in diethyl ether at −78 °C, (thiosulfuric acid) can be obtained. It is a somewhat strong acid with pK<sub>a</sub>s of 0.6 and 1.7 for the first and second dissociations, respectively. Under normal conditions, acidification of solutions of this salt excess with even dilute acids results in complete decomposition to sulfur, sulfur dioxide, and water: The alkylthiosulfates are susceptible to hydrolysis, affording the thiol. This reaction is illustrated by one synthesis of thioglycolic acid:
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Safety
Sodium thiosulfate has low toxicity. LDLo for rabbits is 4000 mg/kg.