Polyprotiske

Polyprotiske, in chemistry, refers to compounds that can donate more than one proton per molecule during dissolution. The term is often used interchangeably with polyprotic, and it especially describes acids that dissociate in several steps. Monoprotic acids release one proton, while diprotic and triprotic acids release two or three protons, respectively. The ability to donate multiple protons affects the acid’s behavior in solution, including pH, buffering capacity, and titration curves.

In aqueous solution, polyprotiske acids dissociate in a sequence of equilibria. Each step has its own dissociation constant (Ka) or, equivalently, a pKa value. For a diprotic acid H2A, the steps are H2A ⇌ H+ + HA− and HA− ⇌ H+ + A2−, with Ka1 typically larger than Ka2. For triprotic acids like H3A, a third step exists, HA2− ⇌ H+ + A3−, with three pKa values in increasing order. The pH and speciation depend on the entire set of pKa values and the total concentration. In many cases, the first dissociation dominates the pH near the initial region, but progressively higher pKa steps become relevant at higher pH.

Common examples include sulfuric acid (H2SO4), a strong first dissociation and a weak second; phosphoric acid (H3PO4) with pKa1 ≈ 2.15, pKa2 ≈ 7.20, pKa3 ≈ 12.35; and carbonic acid (H2CO3) with pKa1 ≈ 6.35 and pKa2 ≈ 10.33. Polyprotiske acids play key roles in buffering, titration analyses, and environmental and biological systems.