Heat of complex formation of A1(III) and Cd(II) with phytic acid. IX.

The interactions of Cd(II) and A1(III) with phytic acid to form both soluble and insoluble complexes have been studied by calorimetry. The reactions were examined at metal ion: phytate mol ratios ranging from 1-6 in the case of Cd(II) and 1 and 4 in the instance of Al(III). The heats of reactions were endothermic within these mol ratios. The enthalpies of soluble complex formation of Cd(II) are compared to those of other divalent metal cations that were studied under similar conditions. Based on the enthalpies, it is suggested that the order of combining affinity of these metal cations for phytic acid is Cu(II) greater than or equal to Zn(II) greater than or less than Cd(II) greater than Mn(II) greater than Mg(II) greater than Co(II) greater than Ni(II) The heats of precipitations (which includes binding, solvation changes, etc.) to form the insoluble Cd(II)- and Al(III)-phytate complexes were determined. The reactions were endothermic and had enthalpies of 20.2 and 28.7 kcal mol-1, respectively. From measurements of the Cd(II) and Al(III) concentrations, it was calculated that 5.4 mol Cd(II) and 4 mol Al(III) bind per mol of precipitated phytate. Al(III) only formed soluble complexes with phytic acid at mol ratios of metal cation:phytate of 1. This reaction was markedly slower, at least as judged by the thermal decay curves, and had an enthalpy considerably larger in magnitude (6-17-fold) than those of all of the divalent cation-phytate reactions thus far investigated in this manner.