# Thiosulfate

Thiosulfate (IUPAC-recommended spelling; sometimes thiosulphate in British English) is an oxyanion of sulfur with the chemical formula S$_2$O2−3. Thiosulfate also refers to the compounds containing this anion, which are the salts of thiosulfuric acid, such as sodium thiosulfate Na$_2$S$_2$O$_3$ and ammonium thiosulfate (NH$_4$)$_2$S$_2$O$_3$. Thiosulfate salts occur naturally. Thiosulfate rapidly dechlorinates water, and is used to halt bleaching in the paper-making industry. Thiosulfate salts are mainly used for dyeing in textiles, and bleaching of natural substances.

## Structure and bonding

The thiosulfate ion is tetrahedral at the central S atom. The thiosulfate ion has C$_{3v}$ symmetry. The external sulfur atom has a valence of 2 while the central sulfur atom has a valence of 6. The oxygen atoms have a valence of 2. The S-S distance of about 201 pm in sodium thiosulphate is appropriate for a single bond. The S-O distances are slightly shorter than the S-O distances in sulfate.
For many years, the oxidation states of the sulfur atoms in the thiosulfate ion were considered to be +6 as in sulfate and −2 as in sulfide for the central and terminal atoms, respectively. This view precluded the disproportionation reaction of thiosulfate into sulfate and sulfide as a redox mechanism for providing energy to bacteria under anaerobic conditions in sediments because there is no change in oxidation state for either S atom. However, XANES spectroscopy measurements have revealed that the charge densities of the sulfur atoms point towards +5 and −1 oxidation states for the central and terminal S atoms, respectively. This observation is consistent with the disproportionation of thiosulfate into sulfate and sulfide as a redox mechanism freeing up energy from microbial fermentation. Yet another interpretation suggests an oxidation state of +4 for the central S atom and 0 for the terminal atom and an unusually long 'full' S=S double bond between the two.

## Formation

Thiosulfate ion is produced by the reaction of sulfite ion with elemental sulfur, and by incomplete oxidation of sulfides (e.g. pyrite oxidation). Sodium thiosulfate can be formed by disproportionation of sulfur dissolving in sodium hydroxide (similar to phosphorus).

## Reactions

Thiosulfate ions reacts with acids to give sulfur dioxide and various sulfur rings:
    8 S$_2$O2−3 + 16 H$^+$ → 8 SO$_2$ + S$_8$ + 8 H$_2$O
This reaction may be used to generate sulfur colloids and demonstrate the Rayleigh scattering of light in physics. If white light is shone from below, blue light is seen from sideways and orange light from above, due to the same mechanisms that color the sky at midday and dusk.
Thiosulfate ions react with iodine to give tetrathionate ions:
    2 S$_2$O2−3 + I$_2$ → S$_4$O2−6 + 2 I$^−$
This reaction is key for iodometry. With bromine (X = Br) and chlorine (X = Cl), thiosulfate ions are oxidized to sulfate ions:
    S$_2$O2−3 + 4 X$_2$ + 5 H$_2$O → 2 SO2−4 + 8 X$^−$ + 10 H$^+$

### Reactions with metals and metal ions

Thiosulfate ion extensively forms diverse complexes with transition metals. This reactivity is related to its role in of silver-based photography.
Also reflecting its affinity for metals, thiosulfate ion rapidly corrodes metals in acidic conditions. Steel and stainless steel are particularly sensitive to pitting corrosion induced by thiosulfate ions. Molybdenum improves the resistance of stainless steel toward pitting (AISI 316L hMo). In alkaline aqueous conditions and medium temperature (60 °C), carbon steel and stainless steel (AISI 304L, 316L) are not attacked, even at high concentration of base (30%w KOH), thiosulfate ion (10%w) and in presence of fluoride ion (5%w KF).
In the era of silver-based photography, thiosulfate salts were consumed on a large scale as a "fixer" reagent. This application exploits thiosulfate ion's ability to dissolve silver halides. Sodium thiosulfate, commonly called hypo (from "hyposulfite"), was widely used in photography to fix black and white negatives and prints after the developing stage; modern "rapid" fixers use ammonium thiosulfate as a fixing salt because it acts three to four times faster.
Thiosulfate salts have been used to extract or leach gold and silver from their ores as a less toxic alternative to cyanide ion.

## Biochemistry

The enzyme rhodanase (thiosulfate sulfurtransferase) catalyzes the detoxification of cyanide ion by thiosulfate ion by transforming them into thiocyanate ion and sulfite ion:
    CN$^−$ + S$_2$O2−3 → SCN$^−$ + SO2−3
Sodium thiosulfate has been considered as an empirical treatment for cyanide poisoning, along with hydroxocobalamin. It is most effective in a pre-hospital setting, since immediate administration by emergency personnel is necessary to reverse rapid intracellular hypoxia caused by the inhibition of cellular respiration, at complex IV.
It activates thiosulfate sulfurtransferase (TST) in mitochondria. TST is associated with protection against obesity and type II (insulin resistant) diabetes.
Thiosulfate can also work as electron donor for growth of bacteria oxidizing sulfur, such as Chlorobium limicola forma thiosulfatophilum. These bacteria use electrons from thiosulfate (and other sources) and carbon from carbon dioxide to synthesize carbon compounds through reverse Krebs cycle.
Some bacteria can metabolise thiosulfates.

## Minerals

Thiosulfate ion is a component of the very rare mineral sidpietersite Pb$_4$(S$_2$O$_3$)O$_2$(OH)$_2$. The presence of this anion in the mineral bazhenovite was disputed.

## Nomenclature

Thiosulfate is an acceptable common name and used almost always.
The functional replacement IUPAC name is sulfurothioate; the systematic additive IUPAC name is trioxidosulfidosulfate(2−) or trioxido-1κ$^3$O-disulfate(S—S)(2−).
Thiosulfate also refers to the esters of thiosulfuric acid, e.g. O,S-dimethyl thiosulfate CH$_3$−O−S(=O)$_2$−S−CH$_3$. Such species are rare.