Inchemistry,phosphorus oxoacid (orphosphorus acid) is a generic name for anyacid whosemolecule consists of atoms ofphosphorus,oxygen, andhydrogen.[1] There is a potentially infinite number of such compounds. Some of them are unstable and have not been isolated, but the derivedanions andorganic groups are present in stablesalts andesters. The most important ones—in biology, geology, industry, and chemical research—are thephosphoric acids, whose esters and salts are thephosphates.
In general, any hydrogen atom bonded to an oxygen atom is acidic, meaning that the –OH group can lose a protonH+
leaving a negatively charged –O−
group and thus turning the acid into aphosphorus oxoanion. Each additional proton lost has an associatedacid dissociation constant Ka1, Ka2 Ka3, ..., often expressed by itscologarithm (pKa1, pKa2, pKa3, ...). Hydrogen atoms bonded directly to phosphorus are generally not acidic.
The phosphorus oxoacids can be classified by theoxidation state(s) of the phosphorus atom(s), which may vary from +1 to +5. The oxygen atoms are usually in oxidation state −2, but may be in state −1 if the molecule includesperoxide groups.
The most important members of this group are thephosphoric acids, where each phosphorus atombonded to four oxygen atoms, one of them through adouble bond, arranged as the corners of atetrahedron. Two or more of thesePO
4 tetrahedra may be connected by shared single-bonded oxygens, forming linear or branched chains, cycles, or more complex structures. The single-bonded oxygen atoms that are not shared are completed with acidic hydrogen atoms. Their generic formula is Hn−x+2PnO3n−x+1, wheren is the number of phosphorus atoms andx is the number offundamental cycles in the molecule's structure.
These acids, and their esters and salts ("phosphates") include some of the best-known and most important compounds of phosphorus.
The simplest member of this class is:
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:
The backbone may be branched, as in:
ThePO
4 tetrahedra may be connected to form closed –P–O– chains, as in:
Metaphosphoric acid is a general term for phosphoric acids with a single cycle, (–P(O)(OH)–O–)n, whoseelemental formula isHPO
3.
Another compound that may be included in this class is
Some phosphorus oxoacids have two or more P atoms in different oxidation states. One example is
