The Collaborative International Dictionary of English v.0.48:

Tribasic \Tri*ba"sic\, a. [Pref. tri- + basic.] (Chem.)
   Capable of neutralizing three molecules of a monacid base, or
   their equivalent; having three hydrogen atoms capable of
   replacement by basic elements on radicals; -- said of certain
   acids; thus, citric acid is a tribasic acid.
   [1913 Webster]