Calorimetric and potentiometric studies on the binding of calcium by phytic acid

Potentiometric as well as thermal titrations of phytic acid and its calcium complexes have been conducted using both the batch and titration microcalorimeters. For phytic acid, the experimental values by either method are in excellent agreement. For the calcium complexes, the total number of groups and the total heat evolved are in agreement but the placement of the curves is different due primarily to the differences in calcium concentration. Binding of calcium by phytic acid is endothermic for the pH range 2.0-12.0 while the heat of dilution of 1 M CaCl2 is exothermic. The binding at pH greater than 11 gives a value of 22.5 +/- 0.9 kcal mol-1. The same enthalpy of binding (22.3 +/- 0.6 kcal mol-1) could be calculated for the entire pH range studied which requires a knowledge of the observed heat of binding, the thermal titration curves of the acid and its calcium complex, and the change in the hydrogen ion environment. Inspection of the thermal binding curves at pH greater than 11 indicates that a number of step-binding constants are involved and that 5.2 mol calcium are bound/mole phytic acid. This value has been confirmed by atomic absorption spectroscopy. Both the thermal and the potentiometric curves are reversible either by the instantaneous injection of acid or base or by continuous titration. Values for the ionization constants (as pK') and the enthalpy of ionization (as delta Hi) have been estimated by computer assisted curve fitting.