Acid–base dissociation constants of 2,2′-bipyridyl in mixed protic solvents

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The acid dissociation constants (Ka), base dissociation constants (Kb) and the autoprotolysis constants (Ks) for 2,2′-bipyridyl in water and in water+alcohol(methanol, ethanol, iso-propanol) mixed solvents have been determined at 25°C and an ionic strength of 0.1 mol l−1, from a direct potentiometric method based on the treatment of the data of a single pH titration. It has been shown that Ka increases, whereas Kb and Ks decrease, with increasing proportion of the alcohol in the mixed solvents. Linear relations between pKa, pKb, pKs and the mole fraction of the alcohol were observed in the composition range investigated. These results are discussed in terms of the properties of solvent and the interactions of the different species existing in dissociation equilibrium with solvents. It is concluded that the higher stabilization of both 2,2′-bipyridyl and its protonated form by dispersion forces and of the proton by its interaction with solvent molecules in the mixed solvents compared with that in water are largely responsible for the observed changes of pKa with composition. On the other hand, the low stabilization of OH− in the mixed solvents relative to that in water and the electrostatic effect are the main factors in determining the solvent effect on pKb.