The isoionic point is the pH value at which a zwitterion molecule has an equal number of 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  and is mainly a term used in protein sciences.
It is different from the isoelectric point (pI) in that pI 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.
- Sørensen S.P.L., Linderstrøm-Lang K., and Lund E (1926). 'The influence of salt concentration on the acid-binding capacity of egg-albuminin'. J. Gen. Physiol., Mar 1927; 8: 543 - 599
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