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 diclofen ac ion and the electrical behavior of diclofenac salts (DM; M = Li, Na , or K) in ethanol/water mixtures. The molar conductance of DM in aque ous solution was measured in a dilute concentration range of <10 mol . m(-3) at 25 degrees C. The Fuoss-Onsager conductance equation for una ssociated electrolytes was used to determine the limiting molar conduc tance (Lambda degrees of DM. The molar conductivity of DM at a finite concentration was, however, larger than the value calculated by the Fu oss-Onsager limiting law. To correct the difference in the experimenta l data, two equations were employed to fit the experimental molar cond uctance of DM. The size of the diclofenac ion was calculated from the limiting molar conductance and compared with the Values obtained by th e Fedors method and the apparent ionization constant (pK(a)) of sodium diclofenac in ethanol/water mixture. The Walden products of DM increa sed up to 20% (w/w) ethanol, but above this concentration of ethanol t hey decreased. Specific interactions between the ions and solvent are discussed.