pentabasic

Century Dictionary and Cyclopedia

1. Noting an acid containing five atoms of hydrogen readily replaceable by metals or electropositive radicals.

Wiktionary

1. adj. chemistry (of an acid) containing five replaceable hydrogen atoms
2. adj. chemistry (of a salt) having five atoms of a univalent metal

GNU Webster's 1913

1. adj. (Chem.) Capable of uniting with five molecules of a monacid base; having five acid hydrogen atoms capable of substitution by a basic radical; -- said of certain acids.