Dithionous acid, H₂S₂O₄

   Although we show the structure of dithionous acid in Table 15.8, only its salts are known and these are powerful reducing agents. Dithionite is prepared by reduction of sulfite in aqueous solution (equation 1.1) by Zn or Na amalgam and possesses eclipsed structure 1.1.

(1.1)

            (1.1)

   The very long S^_S bond in [S₂O₄]^2- (compare r_cov(S) = 103 pm) shows it to be particularly weak and this is consistent with the observation that ³⁵S undergoes rapid exchange between [S₂O]^2- and SO₂ in neutral or acidic solution. The presence of the [SO₂]^- radical anion in solutions of Na₂S₂O₄ has been demonstrated by ESR spectroscopy (see the end of Section 20.7). In aqueous solutions, [S₂O₄]^2- is oxidized by air but in the absence of air, it undergoes reaction 1.2.

                                                    (1.2)