Thermodynamics for the formation of complexes of lanthanide(III) ions with phthalanilic acids

Thermodynamic step-wise stability constants for the interactions of lanthanide(III) ions with phthalanilic, 4-methylphthalanilic, 4-chlorophthalanilic, 4-bromophthalanilic and 4-nitrophthalanilic acids were determined at 22, 30 and 40 ° C by the Bjerrum-Calvin titration technique as adopted by Irving and Rossotti. The order was found to be: Ho(III) ⪢ Dy(III) ⪢ Tb(III) ⪢ Gd(III) < Eu(III) ⪢ Sm(III) ⪢ Nd(III) ⪢ Pr(III) ⪢ La(III), with each anilic acid, while the stabilities of the complexes of different anilic acids with the same metal ion were in the order: phthalanilic acid < 4-methylphthalanilic acid ⪢ 4-chlorophthalanilic acid ⪢ 4-bromophthalanilic acid ⪢ 4-nitrophthalanilic acid. Free energy, enthalpy and entropy changes for the formation of these complexes were also determined which indicated that the chelation was through nitrogen of secondary amide and carboxylate groups.