hydrothionic

from The Century Dictionary.

• Noting an acid discovered by Sehützenberger and first called by him hydrosulphurous acid, now generally called hyposulphurous acid, this latter name having formerly been used for a different substance now known as thiosulphuric, acid, H₂S₂0₃. The formula for Schützenberger's acid is probably H₂S₂O₄. It is an energetic reducing agent.