“…Normally, EDTA is a hexadentate ligand, and the structure of the EDTA complex with metal ion having six coordination numbers may be structurally most compact, since all functional groups of EDTA can coordinate to the metal ion. In the case of metal ion with coordination numbers above 6 such as Ln 3+ , the structure of the EDTA complex may become larger, since a few water molecules coordinated to the metal ion in addition to EDTA (Hoard et al, 1965;Nassimbeni et al, 1979;Templeton et al, 1982Templeton et al, , 1985Nakamura et al, 1995;Sakagami et al, 1999). These speculations imply that the difference in coordination number has a large effect on the diffusion coefficients of M-EDTA (zÀ4)+ for metal ions, but the effect of hydration of the EDTA complex is minimal, which is different from free metal ions.…”