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.

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	pKa₃
Citric^*	3.15	4.78	6.40
Phosphoric	2.12	7.21	12.67