Hyposulphite of soda
3D model (JSmol)
|E number||E539 (acidity regulators, ...)|
CompTox Dashboard (EPA)
|Molar mass||158.11 g/mol (anhydrous) |
248.18 g/mol (pentahydrate)
|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) |
231 g/100 mL (100 °C)
|Solubility||negligible in alcohol|
Refractive index (nD)
|Safety data sheet||External MSDS|
|R-phrases (outdated)||R21 R36 R37 R38|
|NFPA 704 (fire diamond)|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
|what is ?)(|
Sodium thiosulfate (sodium thiosulphate) is an inorganic compound with the formula Na2S2O3.xH2O. Typically it is available as the white or colorless pentahydrate, Na2S2O3·5H2O. The solid is an efflorescent (loses water readily) crystalline substance that dissolves well in water.
Sodium thiosulfate is used in gold mining, water treatment, analytical chemistry, the development of silver-based photographic film and prints, and medicine. The medical uses of sodium thiosulfate include treatment of cyanide poisoning and pityriasis. It is on the World Health Organization's List of Essential Medicines, the safest and most effective medicines needed in a health system.
Sodium thiosulfate is used in the treatment of cyanide poisoning. Other uses include topical treatment of ringworm and tinea versicolor, and treating some side effects of hemodialysis and chemotherapy.
In analytical chemistry, the most important use comes because the thiosulfate anion reacts stoichiometrically with iodine in aqueous solution, reducing it to iodide as the thiosulfate is oxidized to tetrathionate:
- 2 S
3 + I
2 → S
6 + 2 I−
Due to the quantitative nature of this reaction, as well as because Na
3·5H2O has an excellent shelf-life, it is used as a titrant in iodometry. Na
3·5H2O 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.
This application as a photographic fixer was discovered by John Herschel. It is used for both film and photographic paper processing; the sodium thiosulfate is known as a photographic fixer, and is often referred to as 'hypo', from the original chemical name, hyposulphite of soda. Ammonium thiosulfate is typically preferred to sodium thiosulfate for this application.
Sodium thiosulfate and ammonium thiosulfate are a component of an alternative lixiviants to cyanide for extraction of gold. Thiosulfate forms strong soluble complexes with gold(I) ions, [Au(S
2]3−. The advantages of this approach are that (i) thiosulfate is essentially nontoxic and (ii) that ore types that are refractory to gold cyanidation (e.g. carbonaceous or Carlin-type ores) can be leached by thiosulfate. Some problems with this alternative process include the high consumption of thiosulfate, and the lack of a suitable recovery technique, since [Au(S
2]3− does not adsorb to activated carbon, which is the standard technique used in gold cyanidation to separate the gold complex from the ore slurry.
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 (active ingredient in bleach) and in so doing becomes oxidized to sulfate. The complete reaction is:
- 4 NaClO + Na
3 + 2 NaOH → 4 NaCl + 2 Na
4 + H
Similarly, sodium thiosulfate reacts with bromine, removing the free bromine from 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 of the 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.
- 6 NaOH + 4 S → 2 Na
2S + Na
3 + 3 H
- 4 Na
3 → 3 Na
4 + Na
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 pKas of 0.6 and 1.7 for the first and second dissociations, respectively.
3 + 2 HCl → 2 NaCl + S + SO
2 + H
This reaction is known as a "clock reaction", because when the sulfur reaches a certain concentration, the solution turns from colorless to a pale yellow. This reaction has been employed to generate colloidal sulfur. This process is used to demonstrate the concept of reaction rate in chemistry classes.
Aluminium cation reaction
- 2 Al3+ + 3 S
3 + 3 H
2O → 3 SO
2 + 3 S + 2 Al(OH)
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:
2H + Na
3 → Na[O
2H] + NaCl
2H] + H
2O → HSCH
2H + NaHSO
- Record in the GESTIS Substance Database of the Institute for Occupational Safety and Health
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- World Health Organization (2009). Stuart MC, Kouimtzi M, Hill SR (eds.). WHO Model Formulary 2008. World Health Organization. p. 66. hdl:10665/44053. ISBN 9789241547659.
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- Gibson CR (1908). The Romance of Modern Photography, Its Discovery & Its Achievements. Seeley & Co. pp. 37.
hyposulphite-of-soda herschel fixer hypo.
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