The Collaborative International Dictionary of English v.0.48:

Diacid \Di*ac"id\, a. [Pref. di- + acid.] (Chem.)
   Divalent; -- said of a base or radical as capable of
   saturating two acid monad radicals or a dibasic acid. Cf.
   Dibasic, a., and Biacid.
   [1913 Webster]