Thiosulfuric acid, H2S2O3, and polythionates

Thiosulfuric acid may be prepared under anhydrous conditions by reaction 15.110, or by treatment of lead thiosulfate (PbS203) with H2S, or sodium thiosulfate with HCl. The free acid is very unstable, decomposing at 243 K or upon contact with water. 

A representation of the structure of thiosulfuric acid is given in Table 15.8, but the conditions of reaction 15.110 may suggest protonation at sulfur, i.e. (HO)(HS)S02. Thiosulfate salts are far more important than the acid crystallization of the aqueous solution from reaction 15.111 yields Na2S203-5H20. 

Most oxidizing agents (including CI2 and Br2) slowly oxidize [8203] to [804] , and Na28203 is used to remove excess CI2 in bleaching processes. In contrast, I2 rapidly oxidizes [8203] to tetrathionate reaction 15.113 is of great importance in titrimetric analysis. 

Polythionates contain ions of type [8 Og] and may be prepared by condensation reactions such as those in scheme 15.114, but some ions must be made by specific routes. Poly-thionate ions are structurally similar and have two 8031-groups connected by a sulfur chain (15.56 shows [8505] ) solid state structures for a number of salts show chain conformations are variable. In aqueous solution, polythionates slowly decompose to H28O4, 8O2 and sulfur. 

Some compounds are known in which S atoms in a poly-thionate are replaced by Se or Te, e.g. Ba[Se(SS03)2] and Ba[Te(SS03)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. 

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