Thiosulfuric acid and its salts

Production in industry: 2Na₂S₂ + 3O₂ = 2Na₂S₂O₃.

In the laboratory: 1. boiling of sulfites aqueous solution with sulfur powder:

Na₂SO₃ + S = Na₂S₂O₃

2. 4SO₂ + 2H₂S + 6NaOH = 3Na₂S₂O₃ + 5H₂O (strong stirring)

Structure. Thiosulfate, salt of thiosulfuric acid, is a distorted tetrahedron as S—S bond is longer than S—O one. Thiosulfuric acid has two tautomeric forms:

Properties. The strength of thiosulfuric acid is close to H₂SO₄ (K₂ = 2∙10^-2), although it is unstable under normal conditions. Therefore, thiosulfates decompose in acidic medium by the reaction:

Na₂S₂O₃ + H₂SO₄ = Na₂SO₄ + Na₂S₂O₃ = S¯ + Н₂SO₃ = SO₂ + H₂O.

The presence of sulfur (-2) predetermines the reducing properties of thiosulfate. Strong oxidising agents oxidise thiosulfates to elementary S and S (VI) compounds:

Na₂SO₃S^-2 + 4Cl₂⁰ + 5H₂O = 2H₂⁺⁶SO₄ + 2NaCl^-1 + 6HCl.

Na₂SO₃S^-2 + Br₂⁰ + H₂O = S⁰¯ + 2NaBr^-1 + H₂SO₄.

"Soft" oxidants (such as I₂) form tetrathionates:

2Na₂SO₃S + I₂ = Na₂S₄O₆ + 2NaI.

This reaction is widely used in iodometric methods of analysis in analytical chemistry.

PolythioNIC acids. Na₂S₄O₆ is salt of tetrathionic acid.

Preparation. When gaseous Н₂S passes through dilute solution of SО₂ Backenroder liquid is obtained. Its composition includes colloidal sulfur and polythionic acids Н₂S_nO₆, where n = 3-20.

Structure. A chain of elemental sulfur atoms between sp³ -hybridized sulfur is a structure feature of polythionic acids. For example, pentathionic acid:

.

Properties. Unknown in the free state, polythionic acids are strong acids. Relatively stable are their salts with alkali metals. Acid salts are unidentified.

Important. Н₂S₂O₆ is not polythionic acid, since two pyramidal [SO₃]-groups are bound directly through the sulfur atoms. Manganese dithionate can be prepared by passing gaseous SO₂ through MnO₂ suspension:

2SO₂ + MnO₂ = MnS₂O₆

Дата добавления: 2014-01-11