Monoprotisuuden

Monoprotisuuden, or monoproticity, is a property of certain acids in aqueous solution whereby the molecule can donate only one proton (H+) per acid molecule. In a typical monoprotic acid, the dissociation in water is described by the equilibrium HA ⇌ H+ + A− and a single dissociation constant Ka. Because only one proton is removable, the pH behavior and titration of monoprotic acids are comparatively simple.

By contrast, polyprotic acids contain two or more acidic protons that can be released in sequence. For a diprotic acid, for example, the steps are H2A ⇌ H+ + HA− (Ka1) and HA− ⇌ H+ + A2− (Ka2). Polyprotic systems exhibit multiple characteristic pKa values, yielding more complex titration curves and buffering regions than monoprotic acids.

Common examples of monoprotic acids include hydrochloric acid (HCl), nitric acid (HNO3), acetic acid (CH3COOH), formic acid (HCOOH), and benzoic acid (C6H5COOH). Many other inorganic and organic acids behave as monoprotic in typical laboratory pH ranges, though some may act as polyprotic at extreme pH values.

Understanding monoproticity is important for predicting acid strength, buffer capacity, and titration behavior. Monoprotic acids produce a single equivalence point in a titration, whereas polyprotic acids show multiple equivalence points corresponding to successive proton losses. In cases where a second dissociation is very weak (high pKa) within the practical pH range, an acid may be treated as effectively monoprotic for many calculations.