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.

References

Martin, A.N., Swarbrick, J. and Cammarata, A. 1970 Physical Pharmacy, 2nd Edition, Chapter 9 Ionic Equilbria, Tabel 9-1, Lea & Febiger, Philadelphia, PA

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Acid	pKa₁	pKa₂
Carbonic^*	6.37	10.33
Fumaric	3.03	4.38
Succinic	4.19	5.63
Tartaric	3.02	4.36