Sodium thiosulfate

Sodium thiosulfate (sodium thiosulphate) is an inorganic compound with the formula Na₂S₂O₃·(H₂O)(x). Typically it is available as the white or colorless pentahydrate (x = 5). It is a white solid that dissolves well in water. The compound is a reducing agent and a ligand, and these properties underpin its applications.

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."

Medical uses

Sodium thiosulfate is used in the treatment of cyanide poisoning. 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 known as a fixer, 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. It is used for both film and photographic paper processing. Ammonium thiosulfate is typically preferred to sodium thiosulfate for this application. The ability of thiosulfate to dissolve silver ions is related to its ability to dissolve gold ions described below.

Neutralizing chlorinated water

It is used to dechlorinate tap water including lowering chlorine levels for use in aquariums, swimming pools, and spas (e.g., following superchlorination) and within water treatment plants to treat settled backwash water prior to release into rivers. The reduction reaction is analogous to the iodine reduction reaction.

In pH testing of bleach substances, sodium thiosulfate neutralizes the color-removing effects of bleach and allows one to test the pH of bleach solutions with liquid indicators. The relevant reaction is akin to the iodine reaction: thiosulfate reduces the hypochlorite (the active ingredient in bleach) and in so doing becomes oxidized to sulfate. The complete reaction is:

4 NaClO + Na₂S₂O₃ + 2 NaOH → 4 NaCl + 2 Na₂SO₄ + H₂O

Similarly, sodium thiosulfate reacts with bromine, removing the free bromine from the solution. Solutions of sodium thiosulfate are commonly used as a precaution in chemistry laboratories when working with bromine and for the safe disposal of bromine, iodine, or other strong oxidizers.

Structure

Two polymorphs are known as pentahydrate. The anhydrous salt exists in several polymorphs. In the solid state, the thiosulfate anion is tetrahedral in shape and is notionally derived by replacing one of the oxygen atoms by a sulfur atom in a sulfate anion. The S-S distance indicates a single bond, implying that the terminal sulfur holds a significant negative charge and the S-O interactions have more double-bond character.

Production

Sodium thiosulfate is prepared by oxidation of sodium sulfite with sulfur. It is also produced from waste sodium sulfide from the manufacture of sulfur dyes.

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.

6 NaOH + 4 S → 2 Na₂S + Na₂S₂O₃ + 3 H₂O

Principal reactions

Upon heating to 300 °C, it decomposes to sodium sulfate and sodium polysulfide:

4 Na₂S₂O₃ → 3 Na₂SO₄ + Na₂S₅

Thiosulfate salts characteristically decompose upon treatment with acids. Initial protonation occurs at sulfur. When the protonation is conducted in diethyl ether at −78 °C, H₂S₂O₃ (thiosulfuric acid) can be obtained. It is a somewhat strong acid with pK_as 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:

8 Na₂S₂O₃ + 16 HCl → 16 NaCl + S₈ + 8 SO₂ + 8 H₂O

Coordination chemistry

Thiosulfate forms complexes with transition metal ions. One such complex is [Au(S₂O₃)₂]³⁻.

Iodometry

Some analytical procedures exploit the oxidizability of thiosulfate anion by iodine. The reaction produces tetrathionate:

2 S₂O2−3 + I₂ → S₄O2−6 + 2 I⁻

Due to the quantitative nature of this reaction, as well as because Na₂S₂O₃·5H₂O has an excellent shelf-life, it is used as a titrant in iodometry. Na₂S₂O₃·5H₂O is also a component of iodine clock experiments.

This particular use can be set up to measure the oxygen content of water through a long series of reactions in the Winkler test for dissolved oxygen. It is also used in estimating volumetrically the concentrations of certain compounds in solution (hydrogen peroxide, for instance) and in estimating the chlorine content in commercial bleaching powder and water.

Organic chemistry

Alkylation of sodium thiosulfate gives S-alkylthiosulfates, which are called Bunte salts. The alkylthiosulfates are susceptible to hydrolysis, affording the thiol. This reaction is illustrated by one synthesis of thioglycolic acid:

ClCH₂CO₂H + Na₂S₂O₃ → Na[O₃S₂CH₂CO₂H] + NaCl

Na[O₃S₂CH₂CO₂H] + H₂O → HSCH₂CO₂H + NaHSO₄

Safety

Sodium thiosulfate has low toxicity. LDLo for rabbits is 4000 mg/kg.

References