# 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, H_{3}A, and three salt forms, H_{2}A^{-}, HA^{--} and A^{---}. The concentration of each species can be calculated with the equations.

**Equation 1 Fraction present as H**_{3}A

**Equation 2 Fraction present as H**_{2}A^{-}

**Equation 3 Fraction present as HA**^{--}

**Equation 4 Fraction present as A**^{---}

**Some Examples**

**Acid** |
**pKa**_{1} |
**pKa**_{2} |
**pKa**_{3} |

**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

This page (http://www.boomer.org/c/d1/ja/Fig0101/Fig0101.html) was last modified: