M2+•EDTA Binding Affinities: A Modern Experiment in Thermodynamics for the Physical Chemistry Laboratory

Abstract

Isothermal titration calorimetry was used to experimentally determine thermodynamic values for the ethylenediaminetetraacetic acid (EDTA)(aq) + M2+(aq) reactions (M2+ = Ca2+ and Mg2+). Students showed that for reactions in a N-(2-hydroxyethyl)piperazine-N′-ethanesulfonic acid (HEPES) buffer (pH = 7.4), the Mg2+ + EDTA reaction was endothermic, while Ca2+ 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 M2+ ion; thus the observed reaction enthalpies are strongly dependent on pH and buffer ionization enthalpy.