Potentiometric measurements of stoichiometric and apparent affinity constants of EGTA for protons and divalent ions including calcium.

The stoichiometric affinity constants of H+, Ca2+, Mg2+ and Sr+ for the ligand EGTA were determined using a modified version of the pH metric method developed by Moisescu and Pusch (Moisescu, D.G. and Pusch, H. (1975) Pfluegers Arch. 355, 243). The values obtained were slightly higher than those previously published. In addition, the shift in the H+ and Ca2+ stoichiometric constants with ionic strength was found to fit an empirical relationship if the total ionic content of the titration solutions was measured in terms of ionic equivalents, Ie (Johansson, L. (1975) Acta Chem. Scand. A29, 365-373), rather than the formal ionic strength, If. Finally, the apparent affinity of EGTA for Ca2+ ions was measured using an abbreviated form of the titration technique. The measured apparent affinity constant agreed with published results only if calculated with respect to pH (measured) of 7.0, rather than pH (concentration).