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| Names | |
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| IUPAC name Sodium thiosulfate | |
| Other names Sodium hyposulphite Hyposulphite of soda Hypo | |
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| ECHA InfoCard | 100.028.970 |
| EC Number |
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| E number | E539(acidity regulators, ...) |
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| Properties | |
| Na2S2O3 | |
| Molar mass | 158.11 g/mol (anhydrous) 248.18 g/mol (pentahydrate) |
| Appearance | White crystals |
| Odor | Odorless |
| Density | 1.667 g/cm3 |
| Melting point | 48.3 °C (118.9 °F; 321.4 K) (pentahydrate) |
| Boiling point | 100 °C (212 °F; 373 K) (pentahydrate, - 5H2O decomposition) |
| 70.1 g/100 mL (20 °C)[1] 231 g/100 mL (100 °C) | |
| Solubility | negligible inalcohol |
Refractive index (nD) | 1.489 |
| Structure | |
| monoclinic | |
| Hazards | |
| GHS labelling: | |
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| Warning | |
| H315,H319,H335 | |
| P261,P264,P271,P280,P302+P352,P304+P340,P305+P351+P338,P312,P321,P332+P313,P337+P313,P362,P403+P233,P405,P501 | |
| NFPA 704 (fire diamond) | |
| Flash point | Non-flammable |
| Safety data sheet (SDS) | External MSDS |
| Related compounds | |
Othercations | Thiosulfuric acid Lithium thiosulfate Potassium thiosulfate |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Sodium thiosulfate (sodium thiosulphate) is aninorganic compound with the formulaNa2S2O3·xH2O. Typically it is available as the white or colorlesspentahydrate (x = 5), which is a white solid that dissolves well in water. The compound is areducing agent and aligand, and these properties underpin its applications.[2]
Sodium thiosulfate is used predominantly indyeing. 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."[2]
Sodium thiosulfate is used in the treatment ofcyanide poisoning.[3] It is on theWorld Health Organization's List of Essential Medicines.[4][5] Other uses include topical treatment ofringworm andtinea versicolor,[3][6] and treating some side effects ofhemodialysis[7] andchemotherapy.[8][9] In September 2022, the U.S.Food and Drug Administration (FDA) approved sodium thiosulfate under the trade name Pedmark to lessen the risk ofototoxicity andhearing loss in infant, child, and adolescent cancer patients receiving the chemotherapy medicationcisplatin.[10][11]
In photography, sodium thiosulfate is used in bothfilm andphotographic paper processing as afixer, sometimes still called 'hypo' from the original chemical name, hyposulphite of soda.[12] It functions to dissolvesilver halides, e.g.,AgBr, components of photographic emulsions.Ammonium thiosulfate is typically preferred to sodium thiosulfate for this application.[2]
The ability of thiosulfate to dissolve silver ions is related to its ability to dissolve gold ions.
It is used to dechlorinate tap water including loweringchlorine levels for use in aquariums, swimming pools, and spas (e.g., followingsuperchlorination) and withinwater treatment plants to treat settled backwash water prior to release into rivers.[2] The reduction reaction is analogous to the iodine reduction reaction.
InpH testing ofbleach 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 thehypochlorite (the active ingredient inbleach) and in so doing becomes oxidized to sulfate. The complete reaction is:
Similarly, sodium thiosulfate reacts withbromine, 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.
Two polymorphs are known as pentahydrate. The anhydrous salt exists in several polymorphs.[2] In the solid state, thethiosulfateanion is tetrahedral in shape and is notionally derived by replacing one of the oxygen atoms by a sulfur atom in asulfate 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.
Sodium thiosulfate is prepared by oxidation ofsodium sulfite withsulfur.[2] It is also produced from wastesodium sulfide from the manufacture of sulfurdyes.[13]
This salt can also be prepared by boiling aqueoussodium hydroxide and sulfur according to the following equation.[14][15] However, this is not recommended outside of a laboratory, as exposure tohydrogen sulfide can result if improperly handled.
Upon heating to 300 °C, it decomposes tosodium sulfate andsodium polysulfide:
Thiosulfate salts characteristically decompose upon treatment with acids. Initial protonation occurs at sulfur. When the protonation is conducted indiethyl ether at −78 °C,H2S2O3 (thiosulfuric acid) can be obtained. It is a somewhat strong acid with pKas 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 tosulfur,sulfur dioxide, andwater:[13]
Thiosulfate forms complexes withtransition metal ions. One such complex is[Au(S2O3)2]3−.
Someanalytical procedures exploit the oxidizability ofthiosulfate anion byiodine. The reaction producestetrathionate:
Due to the quantitative nature of this reaction, as well as becauseNa2S2O3·5H2O has an excellent shelf-life, it is used as atitrant iniodometry.Na2S2O3·5H2O is also a component ofiodine clock experiments.
This particular use can be set up to measure the oxygen content of water through a long series of reactions in theWinkler 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.
Alkylation of sodium thiosulfate givesS-alkylthiosulfates, which are calledBunte salts.[16] The alkylthiosulfates are susceptible to hydrolysis, affording the thiol. This reaction is illustrated by one synthesis ofthioglycolic acid:
Sodium thiosulfate has low toxicity. LDLo for rabbits is 4000 mg/kg.[2]
hyposulphite-of-soda herschel fixer hypo.
