The orthophosphate ion possesses three sites for attachment of H+. Orthophosphoric acid, H3PO4, has a pKa1 of 2.17, a pKa2 of 7.31, and a pKa3 of 12.36. Because the third hydrogen ion is so difficult to remove from orthophosphate, as evidenced by the very high value of pKa3, very basic conditions are required for PO43- to be present at significant levels in water. It is possible for orthophosphate in natural waters to originate from the hydrolysis of polymeric phosphate species.