Arrhenius Definitions

In the 1880s, Svante Arrhenius defined acids and bases as substances that ionize in solution to produce H⁺ and OH^- ions, respectively. We still often think of acids and bases in these terms, but the definitions can be misleading. For example, an aqueous solution of ammonia, NH₃, contains ammonium and hydroxide ions, NH₄⁺ and OH^-; the Arrhenius definition suggests that these came from the ionization of NH₄OH, but there is no such compound.