 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 (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.