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 |
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Authors: | G H Rounaghi M H Arbab Zavvar K Badiee F Boosaidi M S Kazemi |
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Institution: | 1. Department of Chemistry, Faculty of Sciences, Ferdowsi University of Mashhad, Mashhad, Iran
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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 (logKf) 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}}^{ - } $ . |
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