hypophosphoric

from The Century Dictionary.

• Noting an acid, a distinct tetra basic acid, H₄P₂O₆, produced, along with phosphorous and phosphoric acids, by the slow oxidation of phosphorus in moist air at ordinary temperatures.
• Same as hypophosphorous.

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

• adjective (Chem.) Pertaining to, or derived from, or containing, phosphorus in a lower state of oxidation than in phosphoric compounds.
• adjective (Chem.) an acid, P2H4O6, produced by the slow oxidation of moist phosphorus, and isolated only as a solution in water. It is regarded as a condensation product of one molecule of phosphoric acid with one of phosphorous acid, by partial dehydration.

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

• adjective Of or pertaining to hypophosphoric acid or its derivatives