pKa2

pKa2 is the second acid dissociation constant expressed as a negative logarithm, used for polyprotic acids that can donate more than one proton. For a diprotic acid H2A, the first deprotonation is H2A ⇌ H+ + HA− with Ka1 and pKa1, and the second is HA− ⇌ H+ + A2− with Ka2 and pKa2. The value of pKa2 indicates the strength of the second deprotonation and helps describe how the species HA− and A2− interconvert as pH changes.

In practical terms, pKa2 marks the pH around which the second deprotonation becomes half-complete, meaning [HA−] ≈ [A2−]. Between pKa1 and pKa2, HA− is the dominant form; above pKa2, A2− becomes more prevalent. These pKa values, including pKa2, shape the buffering capacity of solutions and influence the speciation of the acid in different environments.

Common examples include carbonic acid, where pKa1 ≈ 6.35 and pKa2 ≈ 10.33 at 25°C, and phosphoric acid, with pKa1 ≈ 2.15, pKa2 ≈ 7.20, and pKa3 ≈ 12.35. Temperature, ionic strength, and solvent can affect pKa2, as can interactions with metals or binding partners in solution. Understanding pKa2 is essential for predicting pH-dependent behavior, titration curves, and the distribution of charged species in biochemical and environmental contexts.