Abstract
A conductance study of the interaction between Co2+, Ni2+, Cu2+, Cd2+, Zn2+ and Pb2+ ions with cryptands C211, C221 and C222 in different acetonitrile-dimethylsulfoxide mixtures has been carried out at various temperatures. The formation constants of the resulting metal cryptates were determined from the molar conductance-mole ratio data. It was found that the stability of Co2+, Ni2+, Cu2+ and Zn2+ cryptates vary in the order C211>C221>C222, while for Cd2+ cryptates the stability order is C221>C222>C211. A linear relationship is observed between logKf of different metal cryptates and the mole fraction of acetonitrile in the solvent mixture. The enthalpy and entropy of cryptate formation reactions were determined from the temperature dependence of the formation constants. The enthalpy and entropy changes are quite sensitive to the solvent composition and the resultingTΔS0−ΔH0 plot shows a fairly good linear correlation, indicating the existence of an entropy-enthalpy compensation in the cryptate formation reactions.
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