INTERACTIONS BETWEEN CATIONS AND SUGARS .6. CALORIMETRIC METHOD FOR SIMULTANEOUS DETERMINATION OF THE STABILITY CONSTANT AND ENTHALPY CHANGE FOR WEAK COMPLEXATION

A calorimetric method that allows simultaneous determination of both the association constant and the enthalpy change for weak complexation of a cation by a neutral ligand in water is presented. The theoretical and experimental aspects of the procedure are developed. The method involves the separation of the enthalpy of transfer of the electrolyte from water to an aqueous solution of the ligand into a specific contribution characterizing the interaction of the cation with the complexing site of the ligand and a non-specific contribution including all the other interactions of both the cation and the anion with the ligand. The latter contribution is estimated by considering the interactions of the same electrolyte with a non-complexing molecule very similar to the ligand. To illustrate the method the heats of mixing of aqueous solutions of MgCl2, CaCl2, SrCl2, BaCl2 and Ca(ClO4)2 with aqueous solutions of ribose, a complexing ligand, and arabinose, a non-complexing epimer, have been measured at 298.15 K. The stability constant obtained by this calorimetric method for Ca2+ -ribose (beta = 1.9) is in excellent agreement with that found previously by an electrochemical method.