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.
For a diprotic acid, the hydrogen ion concentration can be found at the isoionic point using the following equation[2]
hydrogen ion concentration
first acid dissociation constant
second acid dissociation constant
dissociation constant for water
concentration of the acid
Note that if then and if then . Therefore, under these conditions, the equation simplifies to
The equation can be further simplified to calculate the pH by taking the negative logarithm of both sides to yield
which shows that under certain conditions, the isoionic and
isoelectric point are similar.
References
^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
^Bryan, William P. (1978). "The isoionic point of amino acids and proteins". Biochemical Education. 6: 14–15.
doi:
10.1016/0307-4412(78)90164-4.