Y. Maitani et al., IONIC SIZE AND BEHAVIOR OF DICLOFENAC SALTS IN WATER AND ETHANOL WATER MIXTURES BY CONDUCTIVITY AT 25-DEGREES-C/, Journal of pharmaceutical sciences, 82(12), 1993, pp. 1245-1249
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.