Multiprotic

Multiprotic, also called polyprotic, describes acids or bases that can donate or accept more than one proton per molecule. In aqueous solution, a multiprotic acid dissociates in steps, with successive deprotonations described by Ka1, Ka2, Ka3, and so on. Each step releases one proton and yields a conjugate base with a different strength. Typically, subsequent deprotonations are weaker, so the pKa values increase (for example, phosphoric acid H3PO4 has pKa1 ≈ 2.15, pKa2 ≈ 7.20, pKa3 ≈ 12.35).

Common examples include carbonic acid (H2CO3) and oxalic acid (H2C2O4), which are diprotic, and phosphoric acid (H3PO4) and citric acid (C6H8O7), which are triprotic. Each has multiple ionizable protons, each with its own dissociation constant.

In titration, multiprotic acids exhibit multiple equivalence points corresponding to the sequential removal of protons. Buffers form near each pKa, providing buffering capacity over distinct pH ranges. At the half-equivalence point for a given step, the pH equals the corresponding pKa, reflecting the balance between protonated and deprotonated forms.

The concept is important for understanding protonation states in chemistry and biology. The distribution of species in solution depends on pH and the stepwise equilibria, influencing reactivity, solubility, and interaction with other molecules. While the term is most commonly applied to acids, multiprotic behavior can also describe bases capable of accepting more than one proton.