Isoelectric and Isoionic Points 

Concept #1: Isoelectric or isoionic points represent the pH where a polyprotic acid doesn’t migrate to an electric field because it’s neutral.

Concept #2: Isoelectric and Isoionic points allow us to determine the pH of diprotic and polyprotic systems. 

Example #1: Calculate the isoelectric and isoionic pH of 0.025 M glutamine. pKa1 = 2.19, pKa2 = 9.00. 

Example #2: Draw the structures and charge of aspartic acid at pH = 9.82. 

Practice: Calculate the pI value for histidine. pKa1 (carboxyl group) = 1.60, pKa2 (ammonium group) = 9.28, pKa3 (R-group) = 5.97. 

Practice: Calculate the pI value for gluatamic acid. pKa1 (carboxyl group) = 2.16, pKa2 (ammonium group) = 9.96, pKa3 (R-group) = 4.30.