monoprotic

Monoprotic describes substances that can transfer or accept a single proton (H+) per molecule in acid–base reactions. In the Bronsted–Lowry framework, a monoprotic acid donates one proton: HA → H+ + A−. A monoprotic base accepts one proton: B + H+ → BH+. The defining feature is a single ionizable proton per molecule, in contrast to polyprotic systems that contain two or more ionizable hydrogens.

Monoprotic acids include common species such as hydrochloric acid (HCl), nitric acid (HNO3), and acetic acid (CH3COOH). They have a single acid-dissociation step with one acid dissociation constant, Ka. Monoprotic bases include ammonia (NH3) and pyridine (C5H5N), which accept one proton to form their conjugate acids.

In contrast, polyprotic acids contain multiple ionizable hydrogens, leading to several dissociation steps with distinct Ka values. Examples include sulfuric acid (H2SO4) and phosphoric acid (H3PO4). These multiple steps mean the overall acidity cannot be described by a single Ka value.

In solution, monoprotic acids can be strong, dissociating completely in water, or weak, undergoing partial dissociation. Their pH and concentrations of H+ follow from the single dissociation equilibrium. In titration experiments, monoprotic acids produce a single equivalence point, while polyprotic acids yield two or more, reflecting their multiple protons.