Pharmaceutics Figure 1

Linear Plot of Species versus pH - Triprotic acids


Triprotic acids, such as citric or phosphoric acid, have three pKa values and thus four species at various pH values. These forms are the free acid form, H3A, and three salt forms, H2A-, HA-- and A---. The concentration of each species can be calculated with the equations.

Equation 1 Fraction present as H3A

Equation 2 Fraction present as H2A-

Equation 3 Fraction present as HA--

Equation 4 Fraction present as A---

Some Examples

Acid pKa1 pKa2 pKa3
Citric* 3.15 4.78 6.40
Phosphoric 2.12 7.21 12.67
*Default for Graph above.

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