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Sodium thiosulfate (sodium thiosulphate) is an
inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemist ...
with the formula . Typically it is available as the white or colorless
pentahydrate In chemistry, a hydrate is a substance that contains water or its constituent elements. The chemical state of the water varies widely between different classes of hydrates, some of which were so labeled before their chemical structure was underst ...
, . 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 Photographic film is a strip or sheet of transparent film base coated on one side with a gelatin photographic emulsion, emulsion containing microscopically small light-sensitive silver halide crystals. The sizes and other characteristics of th ...
and
prints In molecular biology, the PRINTS database is a collection of so-called "fingerprints": it provides both a detailed annotation resource for protein families, and a diagnostic tool for newly determined sequences. A fingerprint is a group of conserve ...
, and medicine. The medical uses of sodium thiosulfate include treatment of cyanide poisoning and
pityriasis Pityriasis commonly refers to flaking (or scaling) of the skin. The word comes from the Greek πίτυρον "bran". Classification Types include: * Pityriasis alba * Pityriasis lichenoides chronica * Pityriasis lichenoides et varioliformis acut ...
. It is on the World Health Organization's List of Essential Medicines.


Uses

Sodium thiosulfate is used predominantly in industry. For example, it is used to convert dyes to their soluble colorless forms, which are called leuco. 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. Other uses include topical treatment of ringworm and tinea versicolor, and treating some side effects of hemodialysis and chemotherapy.


Photographic processing

Silver halide A silver halide (or silver salt) is one of the chemical compounds that can form between the element silver (Ag) and one of the halogens. In particular, bromine (Br), chlorine (Cl), iodine (I) and fluorine (F) may each combine with silver to prod ...
s, e.g., AgBr, typical components of photographic emulsions, dissolve upon treatment with aqueous thiosulfate.This application as a photographic fixer was discovered by
John Herschel Sir John Frederick William Herschel, 1st Baronet (; 7 March 1792 – 11 May 1871) was an English polymath active as a mathematician, astronomer, chemist, inventor, experimental photographer who invented the blueprint and did botanical wor ...
. It is used for both
film A film also called a movie, motion picture, moving picture, picture, photoplay or (slang) flick is a work of visual art that simulates experiences and otherwise communicates ideas, stories, perceptions, feelings, beauty, or atmosphere ...
and
photographic paper Photographic paper is a paper coated with a light-sensitive chemical formula, like photographic film, used for making photographic prints. When photographic paper is exposed to light, it captures a latent image that is then developed to form a v ...
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.


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 Shock chlorination is a process used in many swimming pools, water wells, springs, and other water sources to reduce the bacterial and algal residue in the water. Shock chlorination is performed by mixing a large amount of sodium hypochlorite, wh ...
) 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 Bleach is the generic name for any chemical product that is used industrially or domestically to remove color (whitening) from a fabric or fiber or to clean or to remove stains in a process called bleaching. It often refers specifically, to ...
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 In chemistry, hypochlorite is an anion with the chemical formula ClO−. It combines with a number of cations to form hypochlorite salts. Common examples include sodium hypochlorite (household bleach) and calcium hypochlorite (a component of ble ...
(the active ingredient in
bleach Bleach is the generic name for any chemical product that is used industrially or domestically to remove color (whitening) from a fabric or fiber or to clean or to remove stains in a process called bleaching. It often refers specifically, to ...
) and in so doing becomes oxidized to sulfate. The complete reaction is: : 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

On an industrial scale, sodium thiosulfate is produced chiefly from liquid waste products of
sodium sulfide Sodium sulfide is a chemical compound with the formula Na2 S, or more commonly its hydrate Na2S·9 H2O. Both the anhydrous and the hydrated salts in pure crystalline form are colorless solids, although technical grades of sodium sulfide are gener ...
or sulfur
dye A dye is a colored substance that chemically bonds to the substrate to which it is being applied. This distinguishes dyes from pigments which do not chemically bind to the material they color. Dye is generally applied in an aqueous solution an ...
manufacture. In the laboratory, this salt can be prepared by heating an aqueous solution of sodium sulfite with
sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...
or by boiling aqueous
sodium hydroxide Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions . Sodium hydroxide is a highly caustic base and alkali ...
and sulfur according to this equation: :


Principal reactions

Upon heating to 300 °C, it decomposes to
sodium sulfate Sodium sulfate (also known as sodium sulphate or sulfate of soda) is the inorganic compound with formula Na2SO4 as well as several related hydrates. All forms are white solids that are highly soluble in water. With an annual production of 6 milli ...
and sodium polysulfide: : 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 Thiosulfuric acid is the inorganic compound with the formula . It has attracted academic interest as a simple, easily accessed compound that is labile. It has few practical uses. Preparation and degradation The acid cannot be made by acidifying a ...
) can be obtained. It is a somewhat strong acid with p''K''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 (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...
,
sulfur dioxide Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a toxic gas responsible for the odor of burnt matches. It is released naturally by volcanic activ ...
, and water: :


Coordination chemistry

Thiosulfate is a potent ligand for soft metal ions. A typical complex is , which features a pair of S-bonded thiosulfate ligands. Sodium thiosulfate and ammonium thiosulfate have been proposed as alternative lixiviants to
cyanide Cyanide is a naturally occurring, rapidly acting, toxic chemical that can exist in many different forms. In chemistry, a cyanide () is a chemical compound that contains a functional group. This group, known as the cyano group, consists of a ...
for extraction of gold. The advantages of this approach are that (i) thiosulfate is far less toxic than cyanide 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 does not
adsorb Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the ''adsorbate'' on the surface of the ''adsorbent''. This process differs from absorption, in which a fl ...
to
activated carbon Activated carbon, also called activated charcoal, is a form of carbon commonly used to filter contaminants from water and air, among many other uses. It is processed (activated) to have small, low-volume pores that increase the surface area avail ...
, which is the standard technique used in gold cyanidation to separate the gold complex from the ore slurry.


Iodometry

In analytical chemistry, the most important use comes because the thiosulfate anion reacts
stoichiometrically Stoichiometry refers to the relationship between the quantities of reactants and products before, during, and following chemical reactions. Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equals ...
with
iodine Iodine is a chemical element with the symbol I and atomic number 53. The heaviest of the stable halogens, it exists as a semi-lustrous, non-metallic solid at standard conditions that melts to form a deep violet liquid at , and boils to a vi ...
in aqueous solution, reducing it to iodide as the thiosulfate is oxidized to tetrathionate: : Due to the quantitative nature of this reaction, as well as because has an excellent shelf-life, it is used as a titrant in
iodometry Iodometry, known as iodometric titration, is a method of volumetric chemical analysis, a redox titration where the appearance or disappearance of elementary iodine indicates the end point. Note that iodometry involves indirect titration of iodi ...
. 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 The Winkler test is used to determine the concentration of dissolved oxygen in water samples. Dissolved oxygen (D.O.) is widely used in water quality studies and routine operation of water reclamation facilities to analyze its level of oxygen satura ...
. 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.


Aluminium cation reaction

Sodium thiosulfate is used in analytical chemistry. It can, when heated with a sample containing aluminium cations, produce a white precipitate: :


Organic chemistry

Alkylation of sodium thiosulfate gives ''S''-alkylthiosulfates, which are called
Bunte salt In organosulfur chemistry, a Bunte salt is an archaic name for salts with the formula RSSO3–Na+. They are also called S-alkylthiosulfates or S-arylthiosulfates. These compounds are typically derived from alkylation on the pendant sulfur of sodi ...
s. The alkylthiosulfates are susceptible to hydrolysis, affording the thiol. This reaction is illustrated by one synthesis of
thioglycolic acid Thioglycolic acid (TGA) is the organic compound HSCH2CO2H. TGA is often called mercaptoacetic acid (MAA). It contains both a thiol ( mercaptan) and carboxylic acid functional groups. It is a colorless liquid with a strongly unpleasant odor. TGA i ...
: : :


References

{{Antidotes Thiosulfates Sodium compounds Photographic chemicals Antidotes Antifungals for dermatologic use Orphan drugs Specialty drugs World Health Organization essential medicines