pKa3

pKa3 is the negative logarithm of the acid dissociation constant for the third deprotonation step of a polyprotic acid in aqueous solution. For a triprotic acid H3A, the successive equilibria are H3A ⇌ H2A− + H+, Ka1; H2A− ⇌ HA2− + H+, Ka2; HA2− ⇌ A3− + H+, Ka3. Therefore pKa3 = −log10(Ka3). The value indicates the pH at which the fully deprotonated form A3− becomes appreciable, and it, together with pKa1 and pKa2, helps describe the overall protonation state across pH.

In practice, a polyprotic system exhibits three buffering regions near pKa1, pKa2, and pKa3. Typically, pKa3 is larger than pKa2, reflecting the increasing difficulty of removing the final proton from the partially charged species. For example, phosphoric acid has pKa1 ≈ 2.15, pKa2 ≈ 7.20, and pKa3 ≈ 12.37 at 25 °C. Values are sensitive to temperature, ionic strength, and the solvent; thus pKa3 is context-dependent.

Applications and interpretation: pKa3 informs speciation and buffering behavior at high pH, influences metal binding and catalysis, and affects the protonation states of biomolecules in certain environments. It is determined experimentally through titration, spectroscopic methods, or computational estimation, with careful control of conditions.

Limitations: some systems show coupled or non-ideal deprotonations, making simple Henderson–Hasselbalch descriptions less accurate near any pKa. Always report the conditions (temperature, ionic strength, solvent) alongside pKa3.