Dithionous Acid and Dithionites

Dithionous acid, H2S2O4, is not obtainable as a pure compound, but several salts of the acid are known. The acid formally contains sulfur(III), and this suggests reduction of sulfites as a method of preparation. Such a process is illustrated in the following equation  

The structure of the S2042- ion as it occurs in metal salts possesses one C2 axis and two mirror planes that intersect along it so the S2042- ion has C2v symmetry  

In the presence of water, decomposition of S2O42 occurs according to the equation 

Dithionites also decompose to produce radical anions  

Air will oxidize the dithionite ion (which allows it to be used as an oxygen trap), and many metals are reduced to the zero oxidation state by the ion. 

---