In order to elucidate the extraction-ability and -selectivity of a crown ether for metal ions from the standpoint of equilibrium, it is necessary to analyze the overall extraction equilibrium of a metal salt with the crown ether by the fundamental equilibria. It can be considered that the overall extraction equilibrium of a monovalent metal salt (MA) with an electroneutral crown ether (L) into an organic solvent of low dielectric constant consists of four underlying equilibria. Among these basic equilibrium constants, it is next to impossible to determine the ion-pair formation constant in water and the distribution constant of the MLA 1:1:1 complex, because both the ML + and MLA complexes are not stable in water. It has been reported that the ion-pair formation constant in water of a crown ethermonovalent metal ion complex with an anion can be determined by solvent extraction.1,2 Even the ion-pair formation constant in water of a highly lipophilic crown ether-monovalent metal ion complex with an anion can be determined by this method.3 The distribution constant of the MLA complex is calculated from the other three fundamental equilibrium constants and the overall extraction equilibrium constant. By considering the concentrations in the aqueous phase of MLA and MA as well as those of M + and ML + , the actual constants of the overall extraction equilibrium, the aqueous ion-pair formation, and the distribution of the MLA complex are obtained by a successive approximation method. 4 The ion-pair formation constant and the distribution constant provide information about the structure in water and the behavior in organic solvents of the MLA complex, respectively.In this study, actual constants of the overall extraction equilibrium, the distribution for various diluents of low dielectric constant, and the aqueous ion-pair formation of 18-crown-6 (18C6)-alkali metal picrate 1:1:1 complexes were determined at 25˚C to clarify the highest extraction-ability and -selectivity of 18C6 for K + . ExperimentalMaterials 18-Crown-6 (Nisso Co., Ltd.) was recrystallized from acetonitrile and, prior to use, dried at 80˚C in a vacuum. The purity was checked by the melting point (39.2 -40.0˚C). Picric acid, LiOH·H 2 O, NaOH, and KOH were analytical-grade reagents. Rubidium and caesium hydroxides were of reagent grade. The concentrations of solutions of all the alkali metal hydroxides and picric acid were determined by neutralization titration. All of the organic solvents were analyticalgrade reagents. Although 1,2-dichloroethane was purified by distillation, the others were not purified. They were washed several times with distilled water prior to use. 215 ANALYTICAL SCIENCES FEBRUARY 1998, VOL. 14 1998 © The Japan Society for Analytical Chemistry † To whom correspondence should be addressed. 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 complexe...

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