diprotic

Diprotic describes a molecule capable of donating two protons in acid-base reactions, typically in aqueous solution. A diprotic acid, written as H2A, dissociates in two steps: H2A ⇌ H+ + HA− (Ka1) and HA− ⇌ H+ + A2− (Ka2). The two dissociation steps yield two pKa values (pKa1 and pKa2), with pKa1 generally smaller, reflecting the stronger first dissociation. The magnitudes depend on the molecule and solvent; the two steps influence the acid’s strength and buffering behavior.

In solution, a diprotic acid can act as a buffer around both pKa1 and pKa2, stabilizing pH in two distinct regions. In a titration with a base, two equivalence points can appear, corresponding to complete removal of the first and second protons. The relative abundances of H2A, HA−, and A2− depend on pH according to successive Henderson-Hasselbalch relations.

Common examples include carbonic acid (H2CO3), sulfurous acid (H2SO3), and oxalic acid (H2C2O4). Carbonic acid plays a key role in biological bicarbonate buffers and geological CO2–carbonate systems. Many diprotic acids occur in chemistry and biochemistry, and the concept extends to any species with two removable protons. The term contrasts with monoprotic acids (one removable proton) and triprotic acids (three).