Thiosulfuric acid, H₂S₂O₃, and polythionates

   Thiosulfuric acid may be prepared under anhydrous conditions by reaction 1.1, or by treatment of lead thiosulfate (PbS₂O₃) with H₂S, or sodium thiosulfate with HCl. The free acid is very unstable, decomposing at 243K or upon contact with water.

                                   (1.1)

    Arepresentation of the structure of thiosulfuric acid is given in Table 15.8, but the conditions of reaction 1.1 may suggest protonation at sulfur, i.e. (HO)(HS)SO₂. Thiosulfate salts are far more important than the acid; crystallization of the aqueous solution from reaction 1.2 yields Na₂S₂O₃.5H₂O.

                                     (1.2)

(1.1)

    The thiosulfate ion, 1.1, is a very good complexing agent for Ag‏, and Na₂S₂O₃ is used in photography for removing unchanged AgBr from exposed photographic film (equation 1.3 ). In the complex ion [Ag(S₂O₃)₃]^5- , each thiosulfate ion coordinates to Ag‏ through a sulfur donor atom.

                         (1.3)

   Most oxidizing agents (including Cl₂ and Br₂) slowly oxidize [S₂O₃]^2- to [SO₄] ^2-, and Na₂S₂O₃ is used to remove excess Cl₂ in bleaching processes. In contrast, I₂ rapidly oxidizes [S₂O₃] ^2- to tetrathionate; reaction 1.4 is of great importance in titrimetric analysis.

                     (1.4)

    Polythionates contain ions of type [S_nO₆]^2- and may be prepared by condensation reactions such as those in scheme 1.5, but some ions must be made by specific routes. Polythionate ions are structurally similar and have two {SO₃}^- groups connected by a sulfur chain (1.2 shows [S₅O₆]^2-); solid state structures for a number of salts show chain conformations are variable. In aqueous solution, polythionates slowly decompose to H₂SO₄, SO₂ and sulfur.

                              (1.5)

    Some compounds are known in which S atoms in a polythionate are replaced by Se or Te, e.g. Ba[Se(SSO₃)₂] and Ba[Te(SSO₃)₂] . Significantly, Se and Te cannot replace the terminal S atoms, presumably because in their highest oxidation states, they are too powerfully oxidizing and attack the remainder of the chain.

(1.2)