BrønstedLowryAnsatz

The Brønsted-Lowry Ansatz, or Brønsted-Lowry acid-base theory, is a model of acid-base chemistry that defines acids as proton donors and bases as proton acceptors. Proposed independently by Johannes Brønsted and Thomas Martin Lowry in 1923, it describes acid-base reactions as transfers of protons between reaction partners and applies in any solvent or even in the gas phase. A central concept is the idea of conjugate acid-base pairs: when an acid donates a proton, its conjugate base remains; when a base accepts a proton, its conjugate acid forms.

In solution, the theory is illustrated by common reactions such as HCl donating a proton to water to give H3O+ and Cl−, and NH3 acting as a base to form NH4+ in the presence of a proton donor. Water can act as either acid or base, for example NH3 + H2O ⇌ NH4+ + OH−. The strength of an acid is related to its tendency to donate a proton (its acid dissociation constant, Ka), while the strength of a base relates to its tendency to accept a proton (its base dissociation constant, Kb). The strengths are connected through conjugate pairs, and pKa and pKb scales quantify these tendencies.

The Brønsted-Lowry framework is valued for its generality, as it describes proton-transfer reactions beyond water, including nonaqueous solvents and gas-phase systems. It also accounts for amphoterism, where species such as water or ammonium can act as either acids or bases depending on the partner. Limitations exist where proton transfer is not the sole driving force or where solvent effects dominate; nonetheless, the Ansatz remains a foundational concept in modern acid-base chemistry.