Brönsted-Lowry Definitions

The difficulty of fitting NH₃ into a general scheme of acids and bases was solved in 1923 by Johannes Brönsted and Thomas Lowry, who defined an acid as a proton donor and base as a proton acceptor. Thus NH₃ acts as a base and accepts a proton (H⁺) from H₂O, and H₂O acts as an acid in donating a proton to NH₃.

                                      (1.1)

The reverse reaction is also an acid-base reaction: NH₄⁺ acts as an acid in donating a proton to the base, OH^-.  In the Brönsted-Lowry scheme, every acid has a conjugate base and every base has a conjugate acid. Thus NH₄⁺ is the acid conjugate to the base NH₃, and OH^- is the base conjugate to the acid H₂O. Similarly, acetic acid, CH₃COOH, donates its acidic proton to H₂O to produce the conjugate base, acetate ion, CH₃CO₂^-:

                   (1.2)

Notice that water can act both as an acid, with conjugate base OH^-, and as a base, with conjugate acid H₃O⁺.