THE INTERACTIONS OF INOSITOL HEXAPHOSPHATE WITH FE(III) AND CR(III) - A CALORIMETRIC INVESTIGATION .15.

The complexation of Fe(III) and Cr(III) with inositol hexaphosphate (phytic acid) has been studied by calorimetry. The reactions were examined at metal ion: phytate mol ratios 1 and 4, in the instance of Fe(III), and 4 only in the case of Cr(III). The heats of reactions were endothermic within these mol ratios. Precipitation occurred in each instance of the Fe(III)-phytate reactions. From measurements of the remaining Fe(III) and phytic acid concentrations, it was calculated that 4.1 mol Fe(III) bind per mol precipitated phytate at initial mol ratio 4:1. In a similar manner it was found at the initial, lower mol ratio of 1:1 that 3.5 Fe(III) bind per mol precipitated phytate. The heats of precipitations (which include binding, solvation changes, etc.) of the Fe(III)-phytate reactions were approximately 16 kcal mol-1. This value is substantially smaller than those obtained for most previously investigated metal ion-phytate precipitation reactions. Possible reasons for the lower heat of precipitation of the Fe(III)-phytate reaction are discussed. The Cr(III)-phytate reactions were investigated under conditions in which both soluble and insoluble complexes formed. Certain complications, which are detailed, existed in these studies. An unusually large value, i.e., 81 kcal mol-1, was obtained for the heat of precipitation of the insoluble Cr(III)-phytate complex. Due to the above mentioned complications, this enthalpy of precipitation, together with the experimentally determined value of 11.5 mol Cr(III) per mol phytic acid precipitated, are subject to considerable uncertainty. On the basis of the present, and previously reported results (reference 12), it is proposed that Al(III) has a greater affinity for phytic acid than does Fe(III), which in turn exhibits a greater affinity than does Cr(III).