Solvent effect on acidic and basic dissociation constants and thermodynamic parameters of 8-hydroxyquinoline using potentiometric method

The dissociation constants (pK/sub a/), thermodynamic parameters and Gibbs free energies of 8-hydro oxyquinoline are measured by potentiometric method in three mixed aqua-organic media (dioxane, EtOH and MeOH) at 25 deg. C=-0.1 deg. C., the influence of solvents on the dissociation equilibria of OH and N groups has been studied. In case of oxine two dissociation constants are analyzed (acidic and basic). The significance of the relative magnitudes of the values is discussed and some useful generalizations are obtained. The first pK/sub a/ is 4.95 +- 0.01 and the second pK/sub b/ is 9.62 +- 0.00 in 10% dioxane water system, both dissociation constants decrease as dioxane, EtOH and MeOH concentration is increased from 10-50% concentration. The curve is a parabolic one. The values of dissociation constants are calculated by a computer program written in GW-BASIC. From pK values Gibbs free energies are computed and discussed in terms of solute solvent interactions. (author)