p2h4o6

from The Collaborative International Dictionary of English v.0.48
Hypophosphoric \Hy`po*phos*phor"ic\, a. [Pref. hypo- +
   phosphoric.] (Chem.)
   Pertaining to, or derived from, or containing, phosphorus in
   a lower state of oxidation than in phosphoric compounds; as,
   hypophosphoric acid.
   [1913 Webster]

   {Hypophosphoric acid} (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.
      [1913 Webster]
    

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