H2S2O6

from The Collaborative International Dictionary of English v.0.48

Hyposulphuric \Hy`po*sul*phur"ic\, a. [Pref. hypo- + sulphuric.]
   (Chem.)
   Pertaining to, or containing, sulphur in a lower state of
   oxidation than in the sulphuric compounds; as, hyposulphuric
   acid.
   [1913 Webster]

   {Hyposulphuric acid}, an acid, {H2S2O6}, obtained by the
      action of manganese dioxide on sulphur dioxide, and known
      only in a watery solution and in its salts; -- called also
      {dithionic acid}. See {Dithionic}.
      [1913 Webster]

from The Collaborative International Dictionary of English v.0.48

Dithionic \Di`thi*on"ic\, a. [Pref. di- + -thionic.] (Chem.)
   Containing two equivalents of sulphur; as, dithionic acid.
   [1913 Webster]

   {Dithionic acid} (Chem.), an unstable substance, {H2S2O6},
      known only in its solutions, and in certain well-defined
      salts.
      [1913 Webster]