Polythionic acid

Skeletal formula of polythionic acid

Polythionic acid is an oxoacid which has a straight chain of sulfur atoms and has the chemical formula H₂S_nO₆ (n > 2). Trithionic acid (H₂S₃O₆), tetrathionic acid (H₂S₄O₆) are simple examples. The compounds of n < 80 are expected to exist, and those of n < 20 have already been synthesized. Dithionic acid (H₂S₂O₆) does not belong to polythionic acid due to the difference in the property.

Nomenclature

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

• H₂S₂O₆ - dithionic acid
• H₂S₃O₆ - trithionic acid
• H₂S₄O₆ - tetrathionic acid
• H₂S₅O₆ - pentathionic acid, 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 H₂S in aqueous solutions SO₂ (1808). These solution now have 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-pentathionate anion (S₄O₆^2- and S₅O₆^2-, respectively).

Preparation and Properties

In the last few decades in the work of Schmidt and other scientists in Germany, formed a new idea: as H₂S can react with   or HSO₃Cl, forming thiosulfuric acid ḥ, as in the analogous reaction with H₂S₂ formed disulfonmonosulfonic acid ḥ; similarly polysulfur H₂S_n (n = 2-6) gives HS_nSO₃H. Reactions from both ends of the chain polysulfur lead to the formation of polysulfondisulfonic acid HO₃SS_nSO₃H.

There are many known methods for the synthesis of these acids, but the reaction mechanism is unclear because of the large number of simultaneously occurring and competing reactions such as redox, chain transfer, and disproportionation. Typical examples are:

• Interaction hydrogen sulfide and a sulfur dioxide. This yields a complex mixture of various oxyacids of sulfur of different structures, called Wackenroder solution.

H₂S + H₂SO₃ → H₂S₂O₂ + H₂O

H₂S₂O₂ + 2H₂SO₃ → H₂S₄O₆ + 2H₂O

H₂S₄O₆ + H₂SO₃ → H₂S₃O₆ + H₂S₂O₃

• Reactions halosulfate with HSO₃^- or HS₂O₃^-, for example :

SCl₂ + 2HSO₃^- → [O₃SSSO ₃]^2- + 2HCl

S₂Cl₂ + 2HSO₃^- → [O ₃SS₂SO₃]² + 2HCl

SCl₂ + 2HS₂O₃^- → [O₃SS₃SO₃]^2- + 2HCl

• The oxidation of thiosulfate mild oxidizing agents such as I₂, Cu²⁺, S₂O₈^2-, H₂O₂, MnO₂.
• Various special methods of synthesis.

Dithionate ion is obtained by oxidation of aqueous sulfur dioxide powder suspension by manganese or iron oxide (MnO₂, Fe₂O₃):

MnO₂ + 2SO₂ → MnS₂O₆

Trithionate ion synthesized by oxidation of thiosulfate ions by hydrogen peroxide:

2S₂O₃^2- + 4H₂O₂ → S₃O₆^2- + SO₄^2- + 4H₂O

Tetrathionate ion can be obtained by oxidation of thiosulfate ion iodine (reaction is used in iodometry):

S₂O₃^2- + I₂ → S₄O₆^2- + 2I^-

Pentationate ion obtained by the action SCl₂ on the thiosulfate ion and Wackenroder solution by adding to it potassium acetate. Initially fall prismatic crystals of potassium tetrathionate, then pentathionate lamellar crystals of potassium, from which the influence of tartaric acid is an aqueous solution pentathionic acid.

Hexathionate potassium K₂S₆O₆ best synthesize action KNO₂ for K₂S₂O₃ in concentrated HCl at low temperatures.

Anhydrous politionic acids can be found in diethyl ether solution, the following three general ways:

HS_nSO₃H + SO₃ → H₂S_n+2O₆ (n = 1, 2 … 8)

H₂S_n + 2SO₃ → H₂S_n+2O₆ (n = 1, 2 … 8)

2HS_nSO₃H + I₂ → H₂S_2n+2O₆ + 2HI (n = 1, 2 … 6)

More complex polythionates with the number of sulfur atoms is 23, by reacting with thiosulfate SCl₂ and S₂Cl₂.

Polythionic acids with a small number of sulfur atoms in the chain (x = 3 ÷ 6) are the most stable. Polythionic 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 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 of sodium) become sulfites and dithionites.

Occurrence

Polythionic acids are often found in crater lakes. There are various kind of ions containing sulfur atoms derived by hydrogen sulfide and they make the strong acidity condition. It is observed that polythionates in crater lakes are drastically decreased before an eruption occurs. The phenomenon may be useful to predict volcanic activity.

References

• Волынский Н. П., Тиосерная кислота. Политионаты. Реакция Вакенродера. - М.: Наука, 1971. - 80 с.
• Бусев А. И., Симонова Л. Н. Аналитическая химия серы. - М.: Наука, 1975. - 272 с.
• Реми Г. Курс неорганической химии, т.1. М.: Издательство иностранной литературы, 1963. - 921 с.
• Гринвуд Н. Химия элементов: в 2 томах. М.: БИНОМ, 2008. 2 т. - 670 с.