It is one of the major acids used in chemical industry, and is produced by the hydration reaction of phosphorus pentoxide, P4O10. Commercial phosphoric acid is usually of 75-85% purity. The pure acid is a crystalline compound (mp 42.35 oC). One terminal oxygen atom and three OH groups are bonded to the phosphorus atom in the center of a tetrahedron. The three OH groups release protons making the acid tribasic (pK1 = 2.15). When two orthophosphoric acid molecules condense by the removal of an H2O molecule, pyrophosphoric acid, H4P2O7, is formed.