Orthophosphate
Orthophosphoric acid has three hydrogen atoms bonded to oxygen atoms in its structure. All three hydrogens are acidic to varying degrees and can be lost from the molecule as H+ ions (alternatively referred to as protons). When all three H+ ions are lost from orthophosphoric acid, an orthophosphate ion (PO43−) is formed. Orthophosphate is the simplest in a series of phosphates, and is usually just called phosphate by both non-technical people and many chemists alike; see a separate article on phosphate for details.
Because orthophosphoric acid can undergo as many as three dissociations or ionizations (losses of H+ ions), it has three acid dissociation constants called Ka1, Ka2, and Ka3. Another way to provide acid dissociation constant data is to list pKa1, pKa2, and pKa3 instead. Orthophosphate is in a sense the triple conjugate base of phosphoric acid and has three related basicity constants, Kb1, Kb2, and Kb3, which likewise have corresponding pKb1, pKb2, and pKb3 values.
Read more about this topic: Phosphoric Acids And Phosphates