solvent-system concept

Here is a clever generalization that frees acid-base chemistry from water without going all the way to electron pairs. Notice that water quietly ionizes itself: 2 H2O gives H3O+ + OH-, producing a characteristic cation (H3O+) and a characteristic anion (OH-). The solvent-system concept says: in ANY self-ionizing solvent, an acid is whatever increases the concentration of the solvent's characteristic cation, and a base is whatever increases its characteristic anion. Acid plus base then regenerates neutral solvent — the same neutralization idea, generalized.

Apply it to liquid ammonia, which self-ionizes as 2 NH3 gives NH4+ + NH2- (an ammonium cation and an amide anion). By the solvent-system rule, in liquid ammonia an acid is anything that raises NH4+ (so ammonium salts like NH4Cl are 'acids' in ammonia), and a base is anything that raises NH2- (so sodium amide NaNH2 is a 'base'). Neutralization in ammonia is NH4Cl + NaNH2 gives NaCl + 2 NH3 — perfectly parallel to HCl + NaOH gives NaCl + H2O in water. Liquid dinitrogen tetroxide ionizes as N2O4 gives NO+ + NO3-, so nitrosyl compounds act as acids and nitrates as bases. Even BrF3 self-ionizes (2 BrF3 gives BrF2+ + BrF4-), giving an acid-base system in a liquid interhalogen.

The solvent-system concept is powerful precisely where the proton picture breaks down: in aprotic solvents like N2O4 or BrF3, there is no proton to transfer at all, yet a fully coherent acid-base chemistry exists, defined by the solvent's own ions. It is the right tool for non-aqueous and molten-salt chemistry. Its limit, honestly stated, is that it only works for solvents that actually self-ionize, and where they barely do, the Lewis definition is the safer, more general framework. Think of solvent-system as the bridge between Arrhenius (which is just the water case) and the fully general Lewis view.