Phosphorus oxoacids

<h2 id="classification">Classification</h2>
<p>The phosphorus oxoacids can be classified by the <a href="/facts/Oxidation_state/W53W6s6V">oxidation state(s)</a> 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 includes <a href="/facts/Peroxide_group/VMU8y4xB">peroxide groups</a>.
</p>
<h3>Oxidation state +1</h3>

<ul><li><a href="/facts/Hypophosphorous_acid/bmFX2Ga6">Hypophosphorous acid</a> (or phosphinic acid), H3PO2 (or H2PO(OH)), a <a href="/facts/Monoprotic_acid/FySU18n1">monoprotic acid</a> (meaning that only one of the hydrogen atoms is acidic).  Its salts and esters are called <a href="/facts/Hypophosphite/Je5Gckyh">hypophosphites</a> or phosphinates.</li></ul>
<h3>Oxidation state +3</h3>

<ul><li><a href="/facts/Phosphorous_acid/4A4bHBTw">Phosphorous acid</a> (or phosphonic acid), H3PO3 (or HPO(OH)2), a <a href="/facts/Diprotic_acid/FySU18n1">diprotic acid</a> (with only two acidic hydrogens).  Its salts and esters are called <a href="/facts/Phosphite/Uq54qjld">phosphites</a> or phosphonates.</li></ul>
<h3>Oxidation state +4</h3>

<ul><li><a href="/facts/Hypophosphoric_acid/LduLaKG9">Hypophosphoric acid</a>, H4P2O6 (or (HO)2P–P(OH)2). All four hydrogens are acidic. Its salts and esters are <a href="/facts/Hypophosphate/LduLaKG9">hypophosphates</a>.</li></ul>
<h3>Oxidation state +5</h3>
<p>The most important members of this group are the <a href="/facts/Phosphoric_acids_and_phosphates/jAvA75lM">phosphoric acids</a>, where each phosphorus atom <a href="/facts/Chemical_bond/ArNUMnIB">bonded</a> to four oxygen atoms, one of them through a <a href="/facts/Double_bond/qDemMn0c">double bond</a>, arranged as the corners of a <a href="/facts/Tetrahedron/7mkSrl6j">tetrahedron</a>. Two or more of these PO4 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 H<i>n</i>−<i>x</i>+2P<i>n</i>O3<i>n</i>−<i>x</i>+1, where <i>n</i> is the number of phosphorus atoms and <i>x</i> is the number of <a href="/facts/Cycle_basis/B2vXFEeI">fundamental cycles</a> in the molecule's structure.
</p><p>These acids, and their esters and salts ("<a href="/facts/Phosphate/FDGVCxUG">phosphates</a>") include some of the best-known and most important compounds of phosphorus.  
</p>

<p>The simplest member of this class is:
</p>
<ul><li><a href="/facts/Phosphoric_acid/l6NbwyCv">Phosphoric acid</a> proper (also called orthophosphoric acid or monophosphoric acid), H3PO4 (or OP(OH)3), a triprotic acid. It forms orthophosphate salt and esters, commonly called <a href="/facts/Phosphate/FDGVCxUG">phosphates</a>.</li></ul>
<p>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:
</p>
<ul><li><a href="/facts/Pyrophosphoric_acid/QH7FqE52">Pyrophosphoric acid</a>, H4P2O7 (or (HO)2PO–O–PO(OH)2), with four acid hydrogens.  Forms <a href="/facts/Pyrophosphate/L9FPEkhF">pyrophosphates</a>.</li>
<li><a href="/facts/Triphosphoric_acid/hP6qscd2">Triphosphoric acid</a> (or tripolyphosphoric acid), H5P3O10 (or (HO)2P–O–P(OH)–O–P(OH)2), with five acidic hydrogens.  Forms <a href="/facts/Triphosphate/f4ykkLuK">triphosphates</a> or tripolyphosphates.</li>
<li><a href="/facts/Tetraphosphoric_acid/jAvA75lM">Tetraphosphoric acid</a>, H6P4O13 (or (HO)2P(–O–P(OH))2–O–P(OH)2), with six acidic hydrogens.  Forms tetraphosphates.</li></ul>
<p>The backbone may be branched, as in:
</p>
<ul><li>Triphosphono phosphoric acid, H6P4O13 or P(O)(–OP(O)(OH)2)3, a branched isomer of tetrapolyphosphoric acid.</li></ul>
<p>The PO4 tetrahedra may be connected to form closed –P–O– chains, as in:
</p>
<ul><li>Trimetaphosphoric acid (or cyclotriphosphoric acid), H3P3O9 (or (HPO3)3, (–P(O)(OH)–O–)3), a cyclic molecule with three acidic hydrogens. Forms the <a href="/facts/Trimetaphosphate/jAvA75lM">trimetaphosphate</a> salts and esters.</li></ul>
<p><a href="/facts/Metaphosphoric_acid/jAvA75lM">Metaphosphoric acid</a> is a general term for phosphoric acids with a single cycle, (–P(O)(OH)–O–)<i>n</i>, whose <a href="/facts/Elemental_formula/mc8K2H8Y">elemental formula</a> is HPO3.
</p>

<p>Another compound that may be included in this class is
</p>
<ul><li><a href="/facts/Peroxomonophosphoric_acid/Rvb164VW">Peroxomonophosphoric acid</a>, H3PO5 (or OP(OH)2(OOH)), which can be seen as monophosphoric acid with a <a href="/facts/Peroxide/VMU8y4xB">peroxide</a> group replacing the oxygen atom in one of the <a href="/facts/Hydroxyl/1Hs2QAEv">hydroxyl</a> groups</li></ul>
<h3>Mixed oxidation states</h3>
<p>Some phosphorus oxoacids have two or more P atoms in different oxidation states.  One example is
</p>
<ul><li><a href="/facts/Isohypophosphoric_acid/LduLaKG9">Isohypophosphoric acid</a>, H4P2O6 (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</li></ul>

<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/Phosphoramidate/wqcYJvNd">Phosphoramidate</a></li></ul>

<h2 id="further-reading">Further reading</h2>
<ul><li>Schröder HC, Kurz L, Muller WE, Lorenz B (Mar 2000). <a href="https://web.archive.org/web/20110825053648/http://protein.bio.msu.su/biokhimiya/contents/v65/pdf/bcm_0296.pdf">"Polyphosphate in bone"</a> (PDF). <i>Biochemistry (Moscow)</i>. 65 (3): 296–303. <a href="/facts/PMID_(identifier)/JlHAvMHt">PMID</a> <a href="https://pubmed.ncbi.nlm.nih.gov/10739471">10739471</a>. Archived from <a href="http://protein.bio.msu.su/biokhimiya/contents/v65/pdf/bcm_0296.pdf">the original</a> (PDF) on 2011-08-25.</li></ul>
<h2 id="external-links">External links</h2>
<ul><li><a href="https://web.archive.org/web/20050520180549/http://www1.dionex.com/en-us/webdocs/application/industry/chempetro/AN71_V18.pdf"><i>Determination of Polyphosphates Using Ion Chromatography with Suppressed Conductivity Detection, Application Note 71</i> by Dionex</a></li>
<li><a href="https://worldwide.espacenet.com/textdoc?DB=EPODOC&IDX=US3044851">US 3044851</a>, Young, Donald C., "Production of ammonium phosphates and product thereof", published 1962-07-17,  assigned to Collier Carbon & Chemical Co. </li>
<li><a href="https://meshb.nlm.nih.gov/record/ui?name=Phosphorus+Acids">Phosphorus+Acids</a> at the U.S. National Library of Medicine <a href="/facts/Medical_Subject_Headings/C57g2JuF">Medical Subject Headings</a> (MeSH)</li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1"><p>Greenwood, Norman  N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8. <a href="978-0-08-037941-8" target="_blank">978-0-08-037941-8</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li>
</ol>

Phosphorus oxoacids open-in-new

Phosphorus oxoacids