ACID-BASE DISSOCIATION-CONSTANTS OF 2,2'-BIPYRIDYL IN MIXED PROTIC SOLVENTS

The acid dissociation constants (K-a), base dissociation constants (K- b) and the autoprotolysis constants (K-s) for 2,2'-bipyridyl in water and in water + alcohol(methanol, ethanol, iso-propanol) mixed solvents have been determined at 25 degrees 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 K-a increase s, whereas K-b and K-s decrease, with increasing proportion of the alc ohol in the mixed solvents. Linear relations between pK(a), pK(b), pK( s) and the mole fraction of the alcohol were observed in the compositi on range investigated. These results are discussed in terms of the pro perties of solvent and the interactions of the different species exist ing in dissociation equilibrium with solvents. It is concluded that th e 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 large ly responsible for the observed changes of pK(a) with composition. On the other hand, the low stabilization of OH- in the mixed solvents rel ative to that in water and the electrostatic effect are the main facto rs in determining the solvent effect on pK(b). (C) 1998 Elsevier Scien ce B.V. All rights reserved.