The formation constants (logK_MAL^MA) of the complexes of the type (UO₂.A.L] (whereA = IMDA, NTA or EDTA;L = catechol, resorcinol, phloroglucinol, pyrogallol, β-resorcylic acid or protocatechuic acid) have been determined potentiometrically at 25°C and ionic strength,I = 0·2 (mol dm⁻³, NaClO₄) using the Irving-Rossotti approach. The formation constants of the binary complexes (logK_KMA^KM) have been found to lie in the sequence IMDA < NTA < EDTA, whereas those for the mixed ligand complexes (logK_MAL^MA) follow the reverse sequence, IMDA > NTA > EDTA. due to the electrostatic effect.

The variation profiles of certain properties of the lanthanide [Ln(III)] series show discontinuities at or around Gd(III) (4f⁷ stage), Nd (IH)-Pm(III) (4f³−4f⁴ stage) and Ho (III)-Er (III) (4f¹⁰−4f¹¹ stage) due to the presence of a tetrad effect. In the present work the occurrence of the tetradic phenomenon has been studied for the formation constants, log K_MAL^MA of the mixed-ligand Ln (III) chelates of the type [Ln(III).A.L], where Ln(III) = La(III), Ce(III), Pr(III), Nd(III), Sm(III), Eu(III), Gd(III), Tb (III) or Dy(III); A = CDTA or DTPA; L = orcinol, resorcinol or phloroglucinol. The presence of the tetrad effect in log K_MAL^MA values has been tested with the help of the straight line approximation method, differential plot method and inclined-W hypothesis. The magnitude of the effect has been found to lie in the sequence f⁷ > f³ −f⁴.

Formation contants (log K_MAL^MA) of mixed ligand complexes MAL, where M = UO₂²⁺ or Th⁴⁺, A = IMDA, NTA, HEDTA, EDTA, CDTA or DTPA, and L = resorcinol (res), 2-methyl resorcinol (2-Me-res), 5-methyl resorcinol (5-Me-res) or 4-chloro resorcinol (4-Cl-res), have been determined pH-metrically by the Irving-Rossotti approach at 25°C and at an ionic strength,I = 0.2(moldm⁻³KNO₃). The observed stability sequences are IMDA > NTA > HEDTA > EDTA > CDTA > DTPA, and 4-Cl-res > 5-Me-res > 2-Me-res > res with respect to primary and secondary ligands, respectively. Th⁴⁺ forms more stable mixed complexes than UO₂²⁺. The A ΔlogK values are negative due mainly to the charge repulsion involved in the complexation MA + L⇋MAL.