Solution studies of haptens containing azobenzoate and substituted azobenzoate anions in water and in methanol

Abstract

The dissociation constants (pK_d) of o-, m- and p-(4-hydroxyphenylazo)benzoic acid and chloro-substituted 4-hydroxyphenylazobenzoic acids in water at 298.15 K have been obtained by potentiometric titration. pK_d values for azobenzoic acids are higher than those for unsubstituted benzoic acid, suggesting that the 4-hydroxyphenylazo group behaves as an electron-donating group. The dissociation constants of chlorosubstituted 4-hydroxylphenylazobenzoic acids are in excellent agreement with values calculated using the Hammett equation taking as reference pK_d values for unsubstituted 4-hydroxyphenylazobenzoic acids and the substitution constants for chlorine substituents. Hammett substitution constants for 4-hydroxyphenylazo substituents are reported for the first time. Conductances, ion-pair formation constants of o-, m- and p-(4-hydroxyphenylazo)benzoate and substituted azobenzoate salts in water and in methanol at 298.15 K are reported. Stokes radii for these anions in methanol and water are calculated. The data show that these anions are much more solvated in methanol than in water. Solubilities, solubility (ion activity) products and free energies of solution of a number of haptens in water and in methanol are reported. Free-energy data in water and in methanol are combined to calculate the transfer free energies of these electrolytes from water to methanol. A set of single-ion ΔG^°_t values based on the Ph₄AsPh₄B convention for azobenzoate and substituted azobenzoate anions in the water–methanol solvent system is reported.