View source for Polythionic acid

[[File:Polythionic acid 2-D structure.png|250px|thumb|Skeletal formula of polythionic acid]]
'''Polythionic acid''' is an [[oxoacid]] which has a straight chain of [[sulfur]] atoms and has the chemical formula S<sub>n</sub>(SO<sub>3</sub>H)<sub>2</sub> (''n'' + 2 > 2). [[Trithionic acid]] (H<sub>2</sub>S<sub>3</sub>O<sub>6</sub>), [[tetrathionic acid]] (H<sub>2</sub>S<sub>4</sub>O<sub>6</sub>) are simple examples. They are the conjugate acids of [[polythionates]].  The compounds of ''n'' < 80 are expected to exist, and those of ''n'' < 20 have already been synthesized. [[Dithionic acid]] (H<sub>2</sub>S<sub>2</sub>O<sub>6</sub>) 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 SO<sub>3</sub>H-groups. Names of polythionic acids are determined by the number of atoms in the chain of sulfur atoms:

* {{chem|H|2|S|2|O|6}} – [[dithionic acid]]
* {{chem|H|2|S|3|O|6}} – [[trithionic acid]]
* {{chem|H|2|S|4|O|6}} – {{ill|tetrathionic acid|eo|Kvartionata acido|display=1}}
* {{chem|H|2|S|5|O|6}} – {{ill|pentathionic acid|eo|Kvintionata acido|display=1}}, 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]] devoted to the behavior of [[hydrogen sulfide]] in aqueous solutions of [[sulfur dioxide]] (1808). This [[Wackenroder solution|solution]] now has the name of [[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 ({{chem|S|4|O|6|2−}} and {{chem|S|5|O|6|2−}}, respectively).

== Preparation and properties ==
{{chem|H|2|S}} react with {{chem|S|O|3}} or {{chem|H|S|O|3|Cl}}, forming thiosulfuric acid {{Chem|H|2|S|2|O|3}}, as the analogous reaction with {{chem|H|2|S|2}} forms disulfonomonosulfonic acid {{Chem|H|S|2|S|O|3|H}}; similarly polysulfanes H<sub>2</sub>S<sub>''n''</sub> (''n'' = 2–6) give HS<sub>''n''</sub>SO<sub>3</sub>H. Reactions from both ends of the polysulfane chain lead to the formation of polysulfonodisulfonic acid HO<sub>3</sub>SS<sub>''n''</sub>SO<sub>3</sub>H.

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 reaction|redox]], [[chain transfer]], and [[disproportionation]]. Typical examples are:
{{anchor|Wackenroder solution}}
* Interaction between [[hydrogen sulfide]] and [[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]], sulfur dioxide, and [[sulfuric acid]].<ref>{{cite book |last1=Sarkar |first1=Ramaprasad |title=General and inorganic chemistry. |year=2012 |publisher=New Central Book Agency |isbn=9788173817274 |page=483}}</ref>
::H<sub>2</sub>S + H<sub>2</sub>SO<sub>3</sub> → H<sub>2</sub>S<sub>2</sub>O<sub>2</sub> + H<sub>2</sub>O
::H<sub>2</sub>S<sub>2</sub>O<sub>2</sub> + 2&nbsp;H<sub>2</sub>SO<sub>3</sub> → H<sub>2</sub>S<sub>4</sub>O<sub>6</sub> + 2&nbsp;H<sub>2</sub>O
::H<sub>2</sub>S<sub>4</sub>O<sub>6</sub> + H<sub>2</sub>SO<sub>3</sub> → H<sub>2</sub>S<sub>3</sub>O<sub>6</sub> + H<sub>2</sub>S<sub>2</sub>O<sub>3</sub>
* Reactions of sulfur halides with {{chem|HSO|3|−}} or {{chem|HS|2|O|3|−}}, for example :
:: SCl<sub>2</sub> + 2&nbsp;{{chem|HSO|3|−}} → [O<sub>3</sub>SSSO <sub>3</sub>]<sup>2−</sup> + 2&nbsp;HCl
:: S<sub>2</sub>Cl<sub>2</sub> + 2&nbsp;{{chem|HSO|3|−}} → [O<sub>3</sub>SS<sub>2</sub>SO<sub>3</sub>]<sup>2−</sup> + 2&nbsp;HCl
:: SCl<sub>2</sub> + 2&nbsp;{{chem|HS|2|O|3|−}} → [O<sub>3</sub>SS<sub>3</sub>SO<sub>3</sub>]<sup>2−</sup> + 2&nbsp;HCl

Anhydrous polythionic acids can be formed in [[diethyl ether]] solution by the following three general ways:
: HS<sub>n</sub>SO<sub>3</sub>H + SO<sub>3</sub> → H<sub>2</sub>S<sub>''n''+2</sub>O<sub>6</sub> (''n'' = 1, 2 ... 8)
: H<sub>2</sub>S<sub>n</sub> + 2&nbsp;SO<sub>3</sub> → H<sub>2</sub>S<sub>''n''+2</sub>O<sub>6</sub> (''n'' = 1, 2 ... 8)
: 2&nbsp;HS<sub>n</sub>SO<sub>3</sub>H + I<sub>2</sub> → H<sub>2</sub>S<sub>2''n''+2</sub>O<sub>6</sub> + 2&nbsp;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]], [[sulfur dioxide]] and - sometimes - [[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]], [[potassium dichromate]]) polythionic acids and their salts are oxidized to [[sulfate]], and the interaction with strong reducing agents ([[amalgam (chemistry)|amalgam]] of sodium) converts them into [[sulfites]] and dithionites.

==Occurrence==
Polythionic acids are rarely encountered, but [[polythionates|polythion''ates'']] are common and important. 

Polythionic acids have been identified in [[Volcanic crater lake|crater lake]]s.<ref>{{cite journal| author = Takano, B. | title = Correlation of Volcanic Activity with Sulfur Oxyanion Speciation in a Crater Lake | journal = Science | year = 1987 | volume = 235 | issue = 4796 | pages = 1633–1635 | doi=10.1126/science.235.4796.1633 | pmid=17795598| bibcode = 1987Sci...235.1633T | s2cid = 19856265 }}</ref> The phenomenon may be useful to [[prediction of volcanic activity|predict volcanic activity]].

==References==
{{Reflist}}


[[Category:Sulfur oxoacids]]
[[Category:Acids]]
[[Category:Sulfur compounds]]