According to the Arrhenius theory of acids and bases, the acidic species in water is the solvated proton (which we write as H 3 O). This shows that the acidic species is the cation characteristic of the solvent. In water, the basic species is the anion characteristic of the solvent, OH. By extending the Arrhenius defi nitions of acid and base to liquid ammonia, it becomes apparent from Eq. (10.3) that the acidic species is NH 4 and the basic species is NH 2. It is apparent that any substance that leads to an increase in the concentration of NH 4 is an acid in liquid ammonia. A substance that leads to an increase in concentration of NH 2 is a base in liquid ammonia. For other solvents, autoionization (if it occurs) leads to different ions, but in each case presumed ionization leads to a cation and an anion. Generalization of
the nature of the acidic and basic species leads to the idea that in a solvent, the cation characteristic of the solvent is the acidic species and the anion characteristic of the solvent is the basic species. This is known as the solvent concept . Neutralization can be considered as the reaction of the cation and anion from the solvent. For example, the cation and anion react to produce unionized solvent: