A thermodynamic study of complexation of 18-crown-6 with Zn²⁺, Tl⁺, Hg²⁺ and ${\text{UO}}^{{{\text{2 + }}}}_{{\text{2}}}$ cations in acetonitrile-dimethylformamide binary media and study the effect of anion on the stability constant of (18C6-Na⁺) complex in methanol solutions

Abstract

The equilibrium constants and thermodynamic parameters for complex formation of 18-crown-6(18C6) with Zn²⁺, Tl⁺, Hg²⁺ and ${\text{UO}}^{{{\text{2 + }}}}_{{\text{2}}}$ cations have been determined by conductivity measurements in acetonitrile(AN)-dimethylformamide(DMF) binary solutions. 18-crown-6 forms 1:1 complexes [M:L] with Zn²⁺, Hg²⁺ and ${\text{UO}}^{{{\text{2 + }}}}_{{\text{2}}}$ cations, but in the case of Tl⁺ cation, a 1:2 [M:L₂] complex is formed in most binary solutions. The thermodynamic parameters ($\Delta {\text{H}}^{ \circ }_{{\text{c}}}$ and $\Delta {\text{S}}^{ \circ }_{{\text{c}}}$) which were obtained from temperature dependence of the equilibrium constants show that in most cases, the complexes are enthalpy destabilized but entropy stabilized and a non-monotonic behaviour is observed for variations of standard enthalpy and entropy changes versus the composition of AN/DMF binary mixed solvents. The obtained results show that the order of selectivity of 18C6 ligand for these cations changes with the composition of the mixed solvent. A non-linear relationship was observed between the stability constants (logK_f) of these complexes with the composition of AN/DMF binary solutions. The influence of the ${\text{ClO}}^{ - }_{{\text{4}}}$, ${\text{NO}}^{ - }_{{\text{3}}}$ and ${\text{Cl}}^{ - }$ anions on the stability constant of (18C6-Na⁺) complex in methanol (MeOH) solutions was also studied by potentiometry method. The results show that the stability of (18C6-Na⁺) complex in the presence of the anions increases in order: ${\text{ClO}}^{ - }_{{\text{4}}}$ > ${\text{NO}}^{ - }_{{\text{3}}}$ > ${\text{Cl}}^{ - }$.