h2s2o3

from The Collaborative International Dictionary of English v.0.48

Thiosulphuric \Thi`o*sul*phur"ic\, a. [Thio- + sulphuric.]
   (Chem.)
   Of, pertaining to, or designating, an unstable acid,
   {H2S2O3}, analogous to sulphuric acid, and formerly called
   {hyposulphurous acid}.
   [1913 Webster]