http://debianws.lexgopc.com/wiki143/index.php?action=history&feed=atom&title=Isoionic_pointIsoionic point - Revision history2026-05-12T23:08:05ZRevision history for this page on the wikiMediaWiki 1.43.1http://debianws.lexgopc.com/wiki143/index.php?title=Isoionic_point&diff=5572193&oldid=previmported>Lambiam: /* top */ typo2025-06-21T17:04:20Z<p><span class="autocomment">top: </span> typo</p>
<p><b>New page</b></p><div>{{Short description|Term used in protein sciences}}<br />
The '''isoionic point''' is the [[pH]] value at which a [[zwitterion]] [[molecule]] has an equal number of [[Ion|positive and negative charges]] and no adherent ionic species. It was first defined by [[S.P.L. Sørensen]], [[Kaj Ulrik Linderstrøm-Lang]] and [[Ellen Lund]] in 1926 <ref>Sørensen S.P.L., Linderstrøm-Lang K., and Lund E (1926). 'The influence of salt concentration on the acid-binding capacity of egg-albumin'. J. Gen. Physiol., Mar 1927; 8: 543 - 599</ref> and is mainly a term used in [[protein]] sciences.<br />
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It is different from the [[isoelectric point]] (p''I'') in that p''I'' is the pH value at which the net charge of the molecule, ''including'' bound ions is zero. Whereas the isoionic point is at net charge zero in a deionized solution. Thus, the isoelectric and isoionic points are equal when the concentration of charged species is zero.<br />
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For a diprotic acid, the hydrogen ion concentration can be found at the isoionic point using the following equation<ref>{{cite journal|doi=10.1016/0307-4412(78)90164-4 | volume=6 | title=The isoionic point of amino acids and proteins | journal=Biochemical Education | pages=14–15 | last1 = Bryan | first1 = William P.| year=1978 }}</ref><br />
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:<math>[H^+]=\sqrt{{K_1 K_2 C + K_1 K_w} \over {K_1 + C}}</math><br />
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* <math>[H^+]</math> = hydrogen ion concentration<br />
* <math>K_1</math> = first acid dissociation constant<br />
* <math>K_2</math> = second acid dissociation constant<br />
* <math>K_w</math> = dissociation constant for water<br />
* <math>C</math> = concentration of the acid<br />
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Note that if <math>K_1 K_2 C \gg K_1 K_w</math> then <math>K_1 K_2 C + K_1 K_w \approx K_1 K_2 C</math> and if <math>C \gg K_1</math> then <math>K_1 + C \approx C</math>. Therefore, under these conditions, the equation simplifies to<br />
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:<math>[H^+]=\sqrt{{K_1 K_2 C + K_1 K_w} \over {K_1 + C}} \approx \sqrt{{K_1 K_2 C} \over {C}} \approx \sqrt{K_1 K_2}</math><br />
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The equation can be further simplified to calculate the pH by taking the negative logarithm of both sides to yield<br />
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:<math>pH = {{pK_1 + pK_2} \over {2}}</math><br />
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which shows that under certain conditions, the isoionic and [[isoelectric point]] are similar.<br />
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==References==<br />
{{Reflist}}<br />
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[[Category:Zwitterions]]</div>imported>Lambiam