Heats and Entropies of Reaction of Alkaline-earth Metal Ions with Pyrophosphate and Triphosphate Ions

Abstract

The thermodynamic stability constants of the complexes of pyrophosphate and triphosphate ions with Mg (II), Ca (II) and Sr (IL) are evaluated by the pH titration method. The enthalpy changes associated with the formation of the 1 : 1 complexes at 25°C are determined by a direct calorimetric method or calculated from the variation of the thermodynamic stability constants with temperature. From these values of the themodynamic stability constants and enthalpies, the corresponding entropy changes are calculated. The results are as follows (Table 2); (a) in all cases the complex formation is endotherm and entropically favored, (b) the ΔS0 decreases with increasing radius of the metal cations, while the ΔH0 does not, (c) theΔS0 of a triphos- phate complex is always larger than that of the corresponding pyrophosphate complex. All these results are explained in terms of present theories of metal chelate formation in aqueous solution.