THERMODYNAMICS OF COMPLEXATION OF LANTHANIDES AND SOME OF TRANSITION-METAL IONS BY YCLOHEXANE-2-(2-HYDROXYPHENYL)-HYDRAZONO-1,3-DIONE (DCPHD) AND ITS DERIVATIVES

Equilibrium between DCPHD,DC-4-Cl-PHD, and DC-4-Me-PHD and protons, tr ansition, and lanthanide ions have been investigated at 30-degrees-C b y means of potentiometric titration in 75% (v/v) methanol-water mixtur e containing 0.10 M KNO3 as a constant ionic medium. Thermodynamic par ameters (DELTAG, DELTAH and DELTAS) referring to the formation of spec ies HL-, L--, ML(+n-222) and ML2+n-4 (L-- denotes the ligand anion) ha ve been determined in solutions. The solvent effects on the thermodyna mic parameters of the complex formation are discussed in terms of diff erences in the donor ability of methanol and water solvents. The plots of thermodynamic parameters versus ionic potential (Z2/r) of the lant hanide elements is not linear as expected from ionic theory. The obtai ned curve can be resolved in an initial group (the lighter lanthanides ), an intermediate group (Sm-Dy), and a final group (the heavier ones, Tb-Lu). This behavior was explained in terms of differences in the de hydration of lighter lanthanide(III) from that of heavier ones.