), a triprotic acid. It forms orthophosphate salt and esters, commonly called phosphates. The smallest compounds of this class with two or more phosphorus atoms are called "oligophosphoric acids", and the larger ones, with linear –P–O– backbones, are "polyphosphoric acids"; with no definite separation between the two. Some of the most important members are: Pyrophosphoric acid, (or –O–), with four acid hydrogens. Forms pyrophosphates. Triphosphoric acid (or tripolyphosphoric acid), (or –O––O–), with five acidic hydrogens. Forms triphosphates or tripolyphosphates. Tetraphosphoric acid, (or (–O–)2–O–), with six acidic hydrogens. Forms tetraphosphates. The backbone may be branched, as in: Triphosphono phosphoric acid, or P(O)(–)3, a branched isomer of tetrapolyphosphoric acid. The tetrahedra may be connected to form closed –P–O– chains, as in: Trimetaphosphoric acid (or cyclotriphosphoric acid), (or , (–P(O)(OH)–O–)3), a cyclic molecule with three acidic hydrogens. Forms the trimetaphosphate salts and esters. Metaphosphoric acid is a general term for phosphoric acids with a single cycle, (–P(O)(OH)–O–)n, whose elemental formula is . Another compound that may be included in this class is Peroxomonophosphoric acid, H3PO5 (or OP(OH)2(OOH)), which can be seen as monophosphoric acid with a peroxide group replacing the oxygen atom in one of the hydroxyl groups Mixed oxidation states Some phosphorus oxoacids have two or more P atoms in different oxidation states. One example is Isohypophosphoric acid, (or H(OH)(O)P−O−P(O)(OH)2), a tetraprotic acid and isomer of hypophosphoric acid, containing P in oxidation state +3 and +5 See also Phosphoramidate References Further reading External links Determination of Polyphosphates Using Ion Chromatography with Suppressed Conductivity Detection, Application Note 71 by Dionex Dietary minerals Inorganic compounds Phosphates Phosphorus(V) compounds Pyrophosphates Reagents for organic chemistry