pI His = pK a 2 + pK a 3 2 = 6.00 + 9.17 In conclusion, the derivation presented here detailed a simple mathematical approach for determining the isoelectric point (pI) of any amino acid. The zwitterionic form of the amino acid, which is electrically neutral, behaves as an amphoteric species and autoprotolysis equilibrium that thermodynamically and kinetically dominates and determines the pI in solution, provided the corresponding equilibrium constant is higher than other possible acid-base equilibria. Additionally, this approach can be applied to theoretically calculate the pH values of amphoteric solutions, such as the equivalence points in the titration of polyprotic acids (e.g., H3PO4) or polyprotic bases (e.g., Na2CO3) .