According to a definition formulated in 1923 by Johannes Brønsted and Thomas Lowry, an acid is a substance that can do- nate a proton, and a base is a substance that can accept a proton. Under the Brønsted–Lowry definition, an acid–base reaction can be written as An acid (HA) reacts with a base (H 2 O) to form the conjugate base of the acid (A ⫺ ) and the conjugate acid of the base (H 3 O ⫹ ). Accordingly, the ac- etate ion (CH 3 COO ⫺ ) is the conjugate base of acetic acid (CH 3 COOH), and the ammonium ion is the conjugate acid of ammonia (NH 3 ). The acid–base reaction is frequently abbreviated HA Δ H ⫹ ⫹ A ⫺