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]