The thermodynamics of the reactions of ciprofloxacin (CIP) with calcium perchlorate (Ca(ClO4)(2)) and magnesium perchlorate (Mg(ClO4)(2)) have been investigated in water-methanol solvent using conductometric titration. The reactions of CIP with each ion produce two ionic complexes with the general formulas M(CIP)(2+) and M(CIP)(2)(2+). The stability constants K-1 and K-2 at 25 degrees C for the complexes formed from the reaction with Ca(ClO4)(2) were 8.84 x 10(4) and 3.62 x 10(4), respectively. For the reaction with Mg(ClO4)(2) K-1 and K-2 were 1.72 x 10(5) and 2.50 x 10(3), respectively. The enthalpy (Delta H-1 Delta H-2, Delta H-12) and entropy (Delta S-1, Delta S-2, Delta S-12) of complexation reactions were determined from the temperature dependence of the complexation constants. The reactions of CIP with both ions are accompanied by a decrease in entropy (Delta S-12 = -468.12 and -478.89 J/K mol for complexation with Ca(ClO4)(2) and Mg(ClO4)(2), respectively) and enthalpy (Delta H-12= -193.09 and -192.01 kJ/mol for complexation with Ca(ClO4)(2) and Mg(ClO4)(2), respectively), which indicate that the reactions are driven by the enthalpy change. (C) 2011 Elsevier B.V. All rights reserved.