Comments Biprotic acids, such as carbonic or fumaric acid, have two pKa values and thus three species at various pH values. These forms are the free acid form, H2A, and two salt forms, HA-, and A--. The concentration of each species can be calculated with the equations.
Equation 1 Fraction present as H2A
Equation 2 Fraction present as HA-
Equation 3 Fraction present as A--