A thermodynamic study of complex formation between dibenzo-18-crown-6 and dicyclohexano-18-crown-6 with La³⁺ , Hg²⁺, Tl ⁺ and Pb²⁺ in some binary mixed non-aqueous solvents using conductometric method

Abstract

The complextion reactions between La³⁺, Tl⁺ and Pb²⁺ions with the macrocyclicligand, dibenzo-18-crown-6 (DB18C6) and of Hg ⁺ and Tl⁺ ions with dicyclohexa- 18-crown-6 (DC 18C6) have been studied in dimethylformamide (DMF)acetonitrile(AN) and methanol (MeOH)-acetonitrile (AN) binary systems at different temperatures using conductometric method. The conductance data show that the stoichiometry of all of the complexes is 1: 1. The values of thermodynamic parameters (Δ<i>H</i>^o_c<i> </i>and ΔS^o_c) for complexation reactions have been determined from temperature dependence of formation

constants. The obtained results show that the stability of complexes and the thermodynamic parameters (Δ<i>H</i>^o_c<i> </i>, ΔS^o_c) are sensitive to the nature and composition of the mixed solvents. The stability constants of all complexes (except for DB 18C6- Pb²⁺) increase with increasing concentration of solvent with the lower donocity (based on Gutmann doncity scale) in binary mixed systems. The selectivity order of DB 18C6 for metal cations in different concentrations of AN in dimethylformamideacetonitrile binary systems is : Tl⁺>La³⁺> Pb³⁺. In addition, the obtained results show that the stability of DC18C6-Tl⁺ complex at most temperatures is higher than that of DB 18C6-Tl⁺ complex which reflects the effect of substituents on the polyether ring.