Potentiometric Investigation on Mixed-Ligand Complexes of some Lanthanide and Actinide Cations with Hydroxamic Acids and Diamines

The formation of ternary complexes of Th(IV), UO2(II), Ce(III), and La(III) cations with the monohydroxamic acids: acetohydroxamic acid (Aha), benzohydroxamic acid (Bha) and salicylhydroxamic acid (Sham) and diamines: ethylenediamine (en), 1,3-diaminopropane (dap), o-phenylenediamine (o-pda), m-phenylenediamine (m-pda) and p-phenylenediamine (p-pda) was studied using pH-metric titrations at 25 °C and I = 0.10 (NaNO3) in aqueous medium. The stability constants of normal ternary complexes for all the systems studied were evaluated. In the case of dap, protonated binary and ternary complexes could be detected and the corresponding stability constants were determined at the same experimental conditions. Except in the case of aliphatic diamines, there is a good correlation between the basicity of the ligands investigated, and stability of 1:1 binary complexes. Potentiometric measurements show that ternary complexes are formed in a stepwise manner. The order of stability of the ternary complexes in terms of the nature of hydroxamic acid, diamine, and metal ion was investigated and discussed. The relative stability of the mixed-ligand complex was compared with that of the binary complex. In addition, evaluation of the effect of temperature on both the ionization process of the ligands and complex formations for metal-Aha-en and metal-Sham-en ternary systems was studied. The thermodynamic parameters were calculated and discussed.