The proton donor-acceptor concept of acids and bases

7 The proton donor-acceptor concept of acids and bases 

Arrhenius viewed acids and bases as substances which produce hydrogen ions or hydroxide ions on dissociation. As useful a concept as this has been, it did not do a very good job of explaining why NH3, which contains no OH ions, is a base and not an acid, why a solution of FeCls is acidic, or why a solution of Na2S is alkaline. 

The more general working definition of acids and bases we have been using is due to Franklin, who in 1905 developed a theory in which the solvent plays a central role. According to this view, an acid is a solute that gives rise to a cation characteristic of the solvent, and a base is a solute that jdelds a dissolved ion which is also characteristic of the solvent. 

If the solvent is water, these two ions are always H+(aqj and OH (aqj, but in the case of liquid ammonia, which is also a good solvent, the corresponding ions would be NH4 and NH2. That the solvent does play some special role is implied by the self-ionization reactions 

Franklin thus generalized the acid-base concept somewhat, and extended it to non-aqueous solvents. 

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