volume and compressibility effects in the formation of meedta complexes

Kankia B.I, Funck Th., Uedaira H., Buckin V.A.

J. Solution Chemistry, 29, pp. 879-890 (1997)

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 pH12. From  these measurements we obtained the changes in the molar concentration increment of the ultrasonic velocity ΔA, the apparent molar adiabatic compressibility ΔK_SΦ, and the apparent molar volume ΔV_Φ of complex formation. The  hydration  contributions Δ(ΔV_h) to the volume effect of binding range from 39.6 to 46.6 cm³/mol while the hydration contribution to the adiabatic compressibility change in the binding, Δ(ΔK_h),  ranges  from  103.9  x  10^-4  to  131.1  x  10^-4  cm³/(mol bar).  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.