 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

formula_1


Note that if formula_7 then formula_8 and if formula_9 then formula_10.Therefore, under these conditions, the equation simplifies to

formula_11

The equation can be further simplified to calculate the pH by taking the negative logarithm of both sides to yield

formula_12

which shows that under certain conditions, the isoionic and isoelectric point are similar.