The Collaborative International Dictionary of English v.0.48:

Pentacid \Pen*tac"id\, a. [Penta- + acid.] (Chem.)
   Capable of neutralizing, or combining with, five molecules of
   a monobasic acid; having five hydrogen atoms capable of
   substitution by acid residues; -- said of certain complex
   bases.
   [1913 Webster]