dithionic

from The Century Dictionary.

• In chem., an epithet applied to an acid (H₂S₂O₆) formerly called hyposulphuric acid. It is a dibasic acid which cannot be isolated in the pure state, but forms crystallizable salts.

from the GNU version of the Collaborative International Dictionary of English.

• adjective (Chem.) Containing two equivalents of sulphur.
• adjective (Chem.) an unstable substance, H2S2O6, known only in its solutions, and in certain well-defined salts.