Abstract
The interaction between Cd2+ and Pb2+ ions and 18-crown-6 (18C6), 1,10-diaza-18-crown-6 (C22) and 1,7-diaza-15-crown-5 (C21) were studied in water solvent at 25, 35, 45 and 55°С using square wave voltammetric technique. The stoichiometry and stability of the complexes were determined by monitoring the shift in half-wave or peak potential of the polarographic waves of metal against the ligand concentration. Thermodynamic parameters such as ΔG, ΔH and ΔS were obtained by using a polarographic double wall cell in which the temperature could be fixed to ±0.1°C. The results of all experiments show 1 : 1 complexes, but in addition to 1 : 1 ratio, a 2 : 1 ratio of ligand to cation is also obtained for C22–Pb2+ complex. The selectivity order for 18C6 and C22 is Pb2+ > Cd2+. The thermodynamic data ΔG, ΔH and ΔS values show that the complexes are stabilized by both the enthalpy and entropy terms, but C22–Pb2+ complex is stabilized by only enthalpy term.
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