Pharmaceutics Figure 2

Linear Plot of Species versus pH - Biprotic acids


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--

Some Examples

Acid pKa1 pKa2
Carbonic* 6.37 10.33
Fumaric 3.03 4.38
Succinic 4.19 5.63
Tartaric 3.02 4.36
*Default for Graph above.

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