Protophilic solvents, acid-base reactions

As free proton cannot exist alone in solution, reactions in which a proton is split off from an acid to form a conjugate base cannot occur in an isolated system (in a homogeneous solution although this is possible in electrolysis (Section 5.7.1)). The homogeneous solution must contain another base Bn that accepts a proton from the acid HAr (acid HA is, of course, not conjugate with base Bn). It will be seen that this second base can even be the solvent molecule, provided it has protophilic properties. Acid-base reactions thus depend on the exchange of a proton between an acid and a base that are not mutually conjugate ... 

The pyridinium ion (acid 2) as the analyte can be titrated with quaternary ammonium hydroxide (base 3) as it concerns the determination of H+ of the Brensted acid pyridinium, a potentiometric measurement of the pH titration curve and its inflection point is most obvious. In the aprotic, but protophilic, solvent pyridine no stronger acid can exist (see reactions 4.37 and 4.38) than the pyridinium ion itself hence there is a levelling effect but in theory only on the acid side. 

This reaction occurs because the acid possesses some protophilic properties, and these become manifest in the presence of the very strongly protogenic solvent. As may be expected, the stronger the acid is in water, the weaker does it behave as a base in hydrogen fluoride. 

---