# Polythionic acid

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Skeletal formula of polythionic acid

**Polythionic acid** is an [oxoacid](/source/Oxoacid) which has a straight chain of [sulfur](/source/Sulfur) atoms and has the chemical formula Sn(SO3H)2 (*n* > 0). [Trithionic acid](/source/Trithionic_acid) (H2S3O6), [tetrathionic acid](/source/Tetrathionic_acid) (H2S4O6) are simple examples. They are the conjugate acids of [polythionates](/source/Polythionates). The compounds of *n* < 80 are expected to exist, and those of *n* < 20 have already been synthesized.[*[citation needed](https://en.wikipedia.org/wiki/Wikipedia:Citation_needed)*] [Dithionic acid](/source/Dithionic_acid) (H2S2O6) does not belong to the polythionic acids due to strongly different properties.

## Nomenclature

All polythionates anion contains chains of sulfur atoms attached to the terminal SO3H-groups. Names of polythionic acids are determined by the number of atoms in the chain of sulfur atoms:

- H 2S 2O 6 – [dithionic acid](/source/Dithionic_acid)

- H 2S 3O 6 – [trithionic acid](/source/Trithionic_acid)

- H 2S 4O 6 – [tetrathionic acid](/source/Tetrathionic_acid) [[eo](https://eo.wikipedia.org/wiki/Kvartionata_acido)]

- H 2S 5O 6 – [pentathionic acid](https://en.wikipedia.org/w/index.php?title=Pentathionic_acid&action=edit&redlink=1) [[eo](https://eo.wikipedia.org/wiki/Kvintionata_acido)], etc.

## History

Numerous acids and salts of this group have a venerable history, and chemistry systems, where they exist, dates back to the studies [John Dalton](/source/John_Dalton) devoted to the behavior of [hydrogen sulfide](/source/Hydrogen_sulfide) in aqueous solutions of [sulfur dioxide](/source/Sulfur_dioxide) (1808). This [solution](/source/Wackenroder_solution) now has the name of [Heinrich Wilhelm Ferdinand Wackenroder](/source/Heinrich_Wilhelm_Ferdinand_Wackenroder), who conducted a systematic study (1846). Over the next 60–80 years, numerous studies have shown the presence of ions, in particular tetrathionate and pentathionate anion (S 4O2− 6 and S 5O2− 6, respectively).

## Preparation and properties

H 2S react with SO 3 or HSO 3Cl, forming thiosulfuric acid H 2S 2O 3, as the analogous reaction with H 2S 2 forms disulfonomonosulfonic acid HS 2SO 3H; similarly polysulfanes H2S*n* (*n* = 2–6) give HS*n*SO3H. Reactions from both ends of the polysulfane chain lead to the formation of polysulfonodisulfonic acid HO3SS*n*SO3H.

Many methods exist for the synthesis of these acids, but the mechanism is unclear because of the large number of simultaneously occurring and competing reactions such as [redox](/source/Redox_reaction), [chain transfer](/source/Chain_transfer), and [disproportionation](/source/Disproportionation). Typical examples are:

- Interaction between [hydrogen sulfide](/source/Hydrogen_sulfide) and [sulfur dioxide](/source/Sulfur_dioxide) in highly dilute aqueous solution. This yields a complex mixture of various oxyacids of sulfur of different structures, called **Wackenroder solution**. At temperatures above 20 °C solutes slowly decomposes with separation unit [sulfur](/source/Sulfur), sulfur dioxide, and [sulfuric acid](/source/Sulfuric_acid).[1]

- - H2S + H2SO3 → H2S2O2 + H2O - H2S2O2 + 2 H2SO3 → H2S4O6 + 2 H2O - H2S4O6 + H2SO3 → H2S3O6 + H2S2O3

- Reactions of sulfur halides with HSO− 3 or HS 2O− 3, for example :

- - SCl2 + 2 HSO− 3 → [O3SSSO 3]2− + 2 HCl - S2Cl2 + 2 HSO− 3 → [O3SS2SO3]2− + 2 HCl - SCl2 + 2 HS 2O− 3 → [O3SS3SO3]2− + 2 HCl

Anhydrous polythionic acids can be formed in [diethyl ether](/source/Diethyl_ether) solution by the following three general ways:

- HSnSO3H + SO3 → H2S*n*+2O6 (*n* = 1, 2 ... 8)

- H2Sn + 2 SO3 → H2S*n*+2O6 (*n* = 1, 2 ... 8)

- 2 HSnSO3H + I2 → H2S2*n*+2O6 + 2 HI (*n* = 1, 2 ... 6)

Polythionic acids with a small number of sulfur atoms in the chain (*n* = 3, 4, 5, 6) are the most stable. Polythionic acids are stable only in aqueous solutions, and are rapidly destroyed at higher concentrations with the release of [sulfur](/source/Sulfur), [sulfur dioxide](/source/Sulfur_dioxide) and - sometimes - [sulfuric acid](/source/Sulfuric_acid). Acid salts of polythionic acids do not exist. Polythionate ions are significantly more stable than the corresponding acids.

Under the action of oxidants ([potassium permanganate](/source/Potassium_permanganate), [potassium dichromate](/source/Potassium_dichromate)) polythionic acids and their salts are oxidized to [sulfate](/source/Sulfate), and the interaction with strong reducing agents ([amalgam](/source/Amalgam_(chemistry)) of sodium) converts them into [sulfites](/source/Sulfites) and dithionites.

## Occurrence

Polythionic acids are rarely encountered, but [polythion*ates*](/source/Polythionates) are common and important.

Polythionic acids have been identified in [crater lakes](/source/Volcanic_crater_lake).[2] The phenomenon may be useful to [predict volcanic activity](/source/Prediction_of_volcanic_activity).

## References

1. **[^](#cite_ref-1)** Sarkar, Ramaprasad (2012). *General and inorganic chemistry*. New Central Book Agency. p. 483. [ISBN](/source/ISBN_(identifier)) [9788173817274](https://en.wikipedia.org/wiki/Special:BookSources/9788173817274).

1. **[^](#cite_ref-2)** Takano, B. (1987). "Correlation of Volcanic Activity with Sulfur Oxyanion Speciation in a Crater Lake". *Science*. **235** (4796): 1633–1635. [Bibcode](/source/Bibcode_(identifier)):[1987Sci...235.1633T](https://ui.adsabs.harvard.edu/abs/1987Sci...235.1633T). [doi](/source/Doi_(identifier)):[10.1126/science.235.4796.1633](https://doi.org/10.1126%2Fscience.235.4796.1633). [PMID](/source/PMID_(identifier)) [17795598](https://pubmed.ncbi.nlm.nih.gov/17795598). [S2CID](/source/S2CID_(identifier)) [19856265](https://api.semanticscholar.org/CorpusID:19856265).

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Adapted from the Wikipedia article [Polythionic acid](https://en.wikipedia.org/wiki/Polythionic_acid) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Polythionic_acid?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.