The free energies of interaction of ribose, arabinose, lyxose, xylose, glucose, mannose, galactose, talose and allose with CaCl, and KC1 have been determined in water at 298.15 K. The electrochemical method used for these determinations leads to reaction properties, in particular to the association constants, in sufficiently dilute solutions for these properties to be thermodynamically meaningful, unlike n.m.r. methods, which require solutions concentrated both in sugar and in salt. It appears that the constants presented here are distinctly smaller than those determined from n.m.r. data. This work also shows that, depending on the structure of the sugar and on its hydration, the interactions between the sugar and Ca2+ can be either attractive (association) or repulsive (salting-out).

The interaction of the two isomeric pentoses D-(-)-ribose and D-(-)-arabinose with the Ca2' ion in aqueous solution is studied with the help of two types of electrochemical cells (with and without liquid junction) including a liquid-membrane Caz+-selective electrode. Some of the D-(-)-ribose isomers contain a sequence of hydroxyl groups which can interact specifically with the cation. The measurement of the Gibbs free energy of transfer of Ca2+ between water and the sugar solutions allows one to calculate a characteristic pair-interaction parameter. This specific interaction can also be characterized by its association constant: PI = 0.93. The two approaches are studied and their coherence is shown.

. 64, 996 (1986).The thermodynamic parameters characterizing the interaction between Ca2+ and the suitably positioned sequences of hydroxyls of some sugar isomers have been determined. This was done by comparing the properties of D-ribose which bears such sequences of hydroxyls with the properties of D-arabinose chosen as an inactive reference. The enthalpies of solution and of dilution, the apparent molal heat capacities, and the apparent molal volumes of the two pentoses have been first measured in water at 25°C. The measurement of these properties for the transfer of the sugars from water to CaC12 solutions (and, conversely, for the transfer of CaC12 from water to the sugar solutions) directly gives access to the Ca2+-hydroxyls pair interaction parameters. The thermodynamic pro erties of this reaction of association may then be estimated: AH0 = -24 kJ mol-I ; AS0 = -83 J K-' mol-' ; P.The analysis of these data shows that the weak association constant results from a large compensation between the favourable enthalpy and the unfavourable entropy of reaction. JEAN-PIERRE MOREL, CLAUDE LHERMET

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 htter 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 MgCI,, CaCI,, SrCI, , BaCI, and Ca(CIO,), 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 (8= 1.9) is in excellent agreement with that found previously by an electrochemical method.

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