The complexation processes between Cd 2+ cation and a macrocyclic ligand, 1,4,10-trioxa-7,13-diazacyclopentadecane (Kryptofix 21) were studied by the conductometric method in pure acetonitrile and acetonitrile-methanol and acetonitrile-ethyl acetate binary mixtures at different temperatures. In some cases, the stoichiometry of the complex formed between Kryptofix 21 with Cd 2+ cation is 1 : 1 [M L], whereas in some solvent systems 1 : 2 [ML 2 ] and 1 : 3 [ML 3 ] complexes are formed in addition to [ML]. The thermodynamic parameters (ΔS 0 c , ΔH 0 c ) for the formation of the Kryptofix 21-Cd 2+ complex were obtained from the temperature dependence of the stability constant of the [ML] complex. The complexation reaction between Cd 2+ and Kryptofix 21 is athermic or exothermic, depending on the solvent system. 1 The text was submitted by the authors in English.Crown ethers are macrocyclic compounds containing oxygen, sulfur, and/or nitrogen as donor atoms. The first crown ether, dibenzo-18-crown-6, was synthesized in 1967 by Pedersen, and it has gained attention for its ability to form stable complexes with metal cations within its central cavity [1]. With the discovery of crown ethers, the focus on complexation through covalent interactions shifted to that of noncovalent interactions. By accepting metal cations in a more or less "lock and key" fashion, macrocyclic compounds mimic in a relatively uncomplicated way the very complicated functions of biological materials such as enzymes. It is this mimicry that excites the scientists for study of the complexation of crown ethers with various cations in solution; therefore, these compounds can be used as models for ion transport through membranes in biological systems [2]. Study of various macrocyclic compounds in different solvents or solvent mixtures may indicate new approaches for developing pharmaceutical systems or a way to cross the blood organ barrier. Various physicochemical techniques such as spectrophotometry [3], polarography [4], NMR spectrometry [5], calorimetry [6], potentiometry [7], and conductometry [8][9][10][11][12][13][14] have been used to study complex formation between macrocyclic polyethers (crown ethers) and various metal cations in solution. Among these methods, the conductometric technique is a sensitive and inexpensive method with a simple experimental arrangement.The widespread use of nonaqueous solvents began in 1950s in various fields of pure and applied chemistry and has contributed greatly to later advances in chemical science and technology [15]. Numerous data have been published on equilibrium constants and thermodynamic functions of complex formation of crown ethers with metal cations and small organic molecules in pure organic solvents [16,17]. The very small number of data concerning the complex formation in mixed nonaqueous solvents [18,19] was the reason for undertaking this kind of studies in our laboratory. The nature and composition of the solvent system has been found to strongly influence the stoichiometry, selectivity, therm...
