M 2+ •EDTA Binding Affinities: A Modern Experiment in Thermodynamics for the Physical Chemistry Laboratory

Isothermal titration calorimetry was used to experimentally determine thermodynamic values for the ethylenediaminetetraacetic acid (EDTA)(aq) + M 2+ (aq) reactions (M 2+ = Ca 2+ and Mg 2+ ). Students showed that for reactions in a N -(2-hydroxyethyl)piperazine- N ′-ethanesulfonic acid (HEPES) buffer (pH = 7.4), the Mg 2+ + EDTA reaction was endothermic, while Ca 2+ binding to EDTA was exothermic. EDTA is triply ionized at pH 7.4 and therefore must shed a proton to the buffer prior to chelating the M 2+ ion; thus the observed reaction enthalpies are strongly dependent on pH and buffer ionization enthalpy.