Ionic size and behavior of diclofenac salts in water and ethanol/water mixtures by conductivity at 25 degrees C.

The objectives of this study were to evaluate the size of the diclofenac ion and the electrical behavior of diclofenac salts (DM; M = Li, Na, or K) in ethanol/water mixtures. The molar conductance of DM in aqueous solution was measured in a dilute concentration range of < 10 mol.m-3 at 25 degrees C. The Fuoss-Onsager conductance equation for unassociated electrolytes was used to determine the limiting molar conductance (lambda degrees) of DM. The molar conductivity of DM at a finite concentration was, however, larger than the value calculated by the Fuoss-Onsager limiting law. To correct the difference in the experimental data, two equations were employed to fit the experimental molar conductance of DM. The size of the diclofenac ion was calculated from the limiting molar conductance and compared with the values obtained by the Fedors method and the apparent ionization constant (pKa) of sodium diclofenac in ethanol/water mixture. The Walden products of DM increased up to 20% (w/w) ethanol, but above this concentration of ethanol they decreased. Specific interactions between the ions and solvent are discussed.