polyprotoniset

Polyprotoniset, in chemistry often translated as polyprotic, describes substances that can donate or accept more than one proton per molecule. The term is used for acids that have multiple acidic protons and, less commonly, for bases that can accept several protons. In solution, polyprotoniset compounds exist in several protonation states that interconvert as the pH changes.

For polyprotic acids, dissociation occurs in distinct steps, each with its own dissociation constant (pKa). A typical triprotic acid, such as H3A, dissociates first to release the most acidic proton, then successive protons with higher pKa values. For example: H3A ⇌ H+ + H2A− (pKa1), H2A− ⇌ H+ + HA2− (pKa2), HA2− ⇌ H+ + A3− (pKa3). At a given pH, the relative abundances of H3A, H2A−, HA2−, and A3− are determined by these pKa values. Because each step has a different pKa, polyprotic acids can buffer over several pH ranges and may show multiple buffering regions.

Common examples include diprotic acids such as sulfuric acid (H2SO4) and carbonic acid (H2CO3), and triprotic acids such as phosphoric acid (H3PO4) and citric acid. These systems are important in titrations, biochemistry, and environmental chemistry, where multiple proton transfers influence acidity, charge, solubility, and reactivity.

In Finnish texts, the term polyprotoniset is used to refer broadly to these multi-proton donation or acceptance behaviors, with “polyprotic acids” and related concepts forming the standard scientific description.