Well, an oxyacid, is a mineral acid that contains oxygen, and #"hypophosphoric acid"#, like #"sulfuric acid"#, #"perchloric acid"#, and #"nitric acid"# fits this description. It's been a long time since I encountered this particular customer. We would formulate #"hypophosphoric acid"# as #(HO)_2(O=)P-P(=O)(OH)_2#, i.e. a species with a #P-P# bond, and thus a formal #P(+IV)# oxidation state.
This site quotes #pK_(a1)=2.2#, #pK_(a2)=2.8#, #pK_(a3)=7.3#, #pK_(a4)=10.0#. The #P-P# bond length in its salts is #2.19xx10^-10*m#....compare #1.88xx10^-10*m# in #P_4#.
