Abstract
The actual constants of the overall extraction equilibrium, the distribution for various organic solvents having low dielectric constants, and the aqueous ion-pair formation (KMLA) of 18-crown-6 (18C6)-alkali metal (Li - Cs) picrate 1:1:1 complexes were determined at 25°C; the partition constants of 18C6 were also measured at 25°C. The log KMLA values are 2.53±0.21 for Li, 3.29±0.23 for Na, 4.76±0.27 for K, 4.62±0.36 for Rb, and 4.49±0.36 for Cs. The relatively large difference in the log KMLA value between light (Li, Na) and heavy alkali metals (K, Rb, Cs) reflects the difference in the structure of the 18C6-alkali metal ion 1:1 complex between light and heavy alkali metals. Almost all of the partition behavior of 18C6 and its 1:1:1 complexes with alkali metal picrates can be explained by regular solution theory. The molar volumes and solubility parameters of 18C6 and the complexes were determined. For every diluent, the extraction-selectivity order of 18C6 is K > Rb > Cs > Na > Li. The extraction-ability and -selectivity of 18C6 for the alkali metal ion were completely elucidated in terms of the four fundamental equilibria.
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