BrønstedLowrybase

Brønsted-Lowry base is a substance that accepts a proton (H+) in a chemical reaction. The theory, developed independently by Johannes Brønsted and Thomas Lowry in 1923, generalizes acid-base interactions beyond aqueous solutions. In this framework, an acid donates a proton and a base accepts one, with proton transfer constituting the core of the reaction.

A Brønsted-Lowry base forms a conjugate acid after accepting a proton, and the corresponding acid forms a conjugate base after donation. This creates conjugate acid-base pairs that are linked by proton transfer. For example, in the reaction NH3 + H2O ⇌ NH4+ + OH-, ammonia acts as the base and forms the conjugate acid NH4+, while water donates a proton and becomes the conjugate base OH-.

Base strength is solvent-dependent and is measured by the base dissociation constant Kb (or pKb). A larger Kb indicates a stronger base. In water, ammonia has a pKb of about 4.75, reflecting weaker basicity compared with hydroxide ion. For conjugate pairs, Ka × Kb = Kw, where Kw is the autoprotolysis constant of the solvent (in water, Kw ≈ 1.0×10−14 at 25°C). This relation helps compare the strengths of related acid-base pairs.

Beyond water, the Brønsted-Lowry concept applies in nonaqueous solvents and other media, where base strength can differ markedly. The theory underpins many practical topics, including buffer design, organic synthesis, and catalysis, and it complements other acid-base frameworks by focusing on proton transfer rather than solvent-specific behavior alone.