Volume and compressibility effects in the formation of metal-EDTA complexes

Article abstract of DOI:10.1007/BF02768263

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°C and pH 12. From these measurements we obtained the changes in the molar concentration increment of the ultrasonic velocity ΔA, the apparent molar adiabatic compressibility ΔK_sθ, and the apparent molar volume ΔV_θ of complex formation. The hydration contributions Δ(ΔV_h) to the volume effect of binding range from 39.6 to 46.6 cm³-mol^-1 while the hydration contribution to the adiabatic compressibility change in the binding, Δ(ΔK_h), ranges from 103.9 × 10^-4 to 131.1 × 10^-4 cm³-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 Δ(ΔV_h)/ Δ(ΔK_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.