polyprotic

Polyprotic refers to acids or bases that can donate or accept more than one proton. In aqueous solution, such species dissociate in multiple steps, each represented by a separate equilibrium constant (Ka1, Ka2, Ka3, ...). The first dissociation is usually the strongest, and each subsequent step is weaker, so the distribution of protonated forms depends on pH.

Common polyprotic acids include phosphoric acid, H3PO4, which can donate three protons. Its stepwise dissociations have pKa values near 2.15, 7.20, and 12.35, though values vary with temperature and ionic strength. Carbonic acid, H2CO3, is diprotic with pKa1 about 6.35 and pKa2 about 10.33. Sulfurous acid, H2SO3, is another diprotic example with pKa values around 1.8 and 7.2.

In buffer systems, polyprotic acids can maintain buffering capacity over several pH regions because each deprotonation contributes an additional buffering step. They also yield multiple equivalence points in titrations, reflecting successive neutralizations.

Polyprotic behavior is relevant in biology, environmental chemistry, and analytical methods, influencing speciation, solubility, and reaction kinetics. The term also applies to polyprotic bases, species that can accept more than one proton across successive steps, such as carbonate and phosphate bases which convert between CO3^2-, HCO3-, and H2CO3, or PO4^3-, HPO4^2-, and H2PO4-.