The Collaborative International Dictionary of English v.0.48:

Pentathionic \Pen`ta*thi*on"ic\, a. [Penta- + thionic.] (Chem.)
   Pertaining to, or designating, an acid of sulphur obtained by
   leading hydrogen sulphide into a solution of sulphur dioxide;
   -- so called because it contains five atoms of sulphur.
   [1913 Webster]