The determination of binding constants with a differential thermal and potentiometric titration apparatus. II. EDTA, EGTA and calcium.

Abstract

A differential pH and thermal titrimeter has been used to determine the ionization constants of EDTA and EGTA as well as their calcium complexes. The intrinsic heat of binding is a constant for the pH range 2-11.5 for both substances and is found to be -5.4 kcal mol for EDTA and -7.9 kcal mol-1 for EGTA. The binding constants evaluated by fitting to the potentiometric curves and expressed as the log are 10.25 and 11.0, respectively. These values compare reasonably well to those reported in the literature. We have proposed that the binding of calcium occurs even at acid pH based on the pH drop observed at pH 2 when calcium is added, the reversibility of the thermal and the potentiometric curves, the lack of hysteresis upon rapid titration, the constancy of the intrinsic heat of binding and fitting of the entire potentiometric curves using the appropriate binding constants.