VOLUME AND COMPRESSIBILITY EFFECTS IN THE FORMATION OF METAL-EDTA COMPLEXES

摘要

We used precise measurements of ultrasonic velocity and density to study the complexation of ethylendiaminetetraacetic acid (EDTA) with Mg2+, Ca2+, Sr2+, and Ba2+ at 25 degrees C and pH 12. From these measurements we obtained the changes in the molar concentration increment of the ultrasonic velocity Delta A, the apparent molar adiabatic compressibility Delta K-s phi, and the apparent molar volume Delta V-phi of complex formation. The hydration contributions Delta(Delta V-h) to the volume effect of binding range from 39.6 to 46.6 cm(3)-mol(-1) while the hydration contribution to the adiabatic compressibility change in the binding, Delta(Delta K-h), ranges from 103.9 X 10(-4) to 131.1 X 10(-4) cm(3)-mol(-1)-bar(-1). These data are interpreted in terms of dehydration of interacting molecules, i.e., transfer of water molecules from the hydration shells of cations and EDTA into the bulk water. The ratio Delta(Delta V-h)/ Delta(Delta K-h) is in the range 0.35 to 0.38 bar, indicating a dominant contribution from the dehydration of charged atomic groups in the volume and the compressibility effects of complex formation. [References: 38]