hypophosphorous : Idioms & Phrases

• noun a clear or yellow monobasic acid (H3PO2)
  phosphorous acid; orthophosphorous acid.
  

WordNet

• (Chem.) , an acid, H3PO2, whose salts are produced by the action of barium hygrate on phosphorus. It may be obtained from its water solution, by exaporation and freezing, as a white crystalline substance. It is a powerful reducing agent.

Webster 1913