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

We used precise measurements of ultrasonic velocity and density to stu dy the complexation of ethylendiaminetetraacetic acid (EDTA) with Mg2, Ca2+, Sr2+, and Ba2+ at 25 degrees C and pH 12. From these measureme nts we obtained the changes in the molar concentration increment of th e ultrasonic velocity Delta A, the apparent molar adiabatic compressib ility Delta K-s phi, and the apparent molar volume Delta V-phi of comp lex formation. The hydration contributions Delta(Delta V-h) to the vol ume 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 dehydr ation of interacting molecules, i.e., transfer of water molecules from the hydration shells of cations and EDTA into the bulk water. The rat io 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 a tomic groups in the volume and the compressibility effects of complex formation.