Volume Changes and Equilibrium Constants for Protonation of [M(edta)]2−

Abstract The volume changes and the equilibrium constants for the first protonation of [Co(edta)]2−, [Ni(edta)]2−, and [Cu(edta)]2− (H4edta=ethylenediaminetetraacetic acid) have been determined from density and pH measurements at 25 °C. In the case of [Cu(edta)]2−, a second protonation is assumed to successively occur. The volume change and the equilibrium constant for the second protonation have been estimated. The volume changes for the first protonation of Co(II)-, Ni(II)-, and Cu(II)-edta chelates were small and positive (3.5, 3.3, and 4.8 cm3 mol−1, respectively) and that for the second protonation of a Cu(II)-edta chelate was larger than the first one (8 cm3 mol−1).