pentabasic

from The Century Dictionary.

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

from the GNU version of the Collaborative International Dictionary of English.

• adjective (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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective chemistry (of an acid) containing five replaceable hydrogen atoms
• adjective chemistry (of a salt) having five atoms of a univalent metal