Dissociation Constant of Quinic Acid and Association Constants of Some Quinates in Aqueous and in Alcoholic-Aqueous Mixed Solvents

The dissociation constant of D-(-)-quinic acid {(1R,3R,4S,5R)-(-)-1,3,4,5-tetrahydroxycyclohexane-1-carboxylic acid} was determined potentiometrically and conductometrically in aqueous, 50 % (v/v) methanol-water, and 50 % (v/v) ethanol-water mixtures at (293.15, 298.15, 303.15, and 308.15) K. In addition, the association constants of lithium, sodium, and potassium salts of quinic acid were determined conductometrically in the same solvents and at the same temperatures. The conductivity data have been analyzed using both the Lee-Wheaton and Shedlovesky conductivity equations. The molar conductance (Lambda), limiting molar conductance (Lambda(0)), limiting ionic molar conductance (lambda(0)(+/-)), diffusion coefficients (D(0)), ionic mobility (u(+/-)), and transfer numbers of the studied ions (t(+/-)) were calculated and discussed. The association constants (K(A)), Walden products (Lambda(0)eta(0) and lambda(0)eta(0)), hydrodynamic radii (R(H)), fluidity ratio (R), the activation energy of the transport process (E), and the standard thermodynamic parameters of association and dissociation (Delta G degrees, Delta H degrees, and Delta S degrees) were also calculated and discussed in terms of solute-solvent interactions. The effect of hydrogen bonds, relative permittivity, and temperature on the transport properties and the dissociation and the association behavior has been studied and discussed. The results show that the dissociation constant, the molar conductance, and the limiting molar conductance decrease as the relative permittivity of the solvent decreased and as the viscosity of the solvent increased. On the other hand, the association constant was found to increase as the relative permittivity of the solvent decreased and as the viscosity of the solvent increased. In addition, the results show that the molar conductance, the limiting molar conductance, and the dissociation and the association constant values increased as the temperature increased, which indicates that the association and the dissociation processes are endothermic. Also, it was found that the predominant order of the association constant values for all studied systems at the same temperature is in the following order with some exceptions: EtOH-H(2)O > MeOH-H(2)O > H(2)O. This order reverses to that observed for Ao values which can interpreted on the basis of the effect of hydrogen bonding and the relative permittivity which plays an important role in changing the association process.