triprotic

Triprotic is a term used in chemistry to describe acids that can donate three protons in successive dissociation reactions. In water, a triprotic acid H3A dissociates in three steps: H3A -> H2A- + H+, with equilibrium constant Ka1; H2A- -> HA2- + H+, Ka2; HA2- -> A3- + H+, Ka3. Each step has its own pKa value (pKa1 = −log10 Ka1, etc.). The three pKa values often differ by orders of magnitude, but in some acids they are closer.

Common examples include phosphoric acid (H3PO4) and citric acid (H3C6H5O7). These acids exhibit three distinct buffering regions around pKa1, pKa2, and pKa3. The fully protonated form H3A dominates at very low pH; the fully deprotonated form A3- dominates at high pH, with amphiprotic intermediates H2A- and HA2- present in between.

In titration and buffering contexts, the triprotic system can provide up to three buffering plateaus or regions, yielding two or three observable inflection points depending on the spacing of the pKa values. Many biological systems exploit triprotic equilibria, such as the phosphate buffering system in cells and body fluids (H3PO4/H2PO4-/HPO4^2-/PO4^3-) and citric acid in metabolic processes.

Not all polyprotic acids are triprotic; there are monoprotic, diprotic, and polyprotic species. The term triprotic is most often used to refer to acids that can donate three protons, and it is a useful descriptor in acid-base chemistry, biochemistry, and analytical chemistry.