Extraction of Alkali Metal Picrates with Benzo-18-crown-6 into Various Organic Solvents. Fundamental Equilibria which Govern the Highest Extraction Selectivity for K+ among Alkali Metal Ions

Abstract

Abstract For the determination of the actual overall extraction equilibrium constant (Kex) and the ion-pair formation constant (KMLA) in water of a crown ether–metal salt 1 : 1 : 1 complex, a more completed method is established by considering the concentration of the resulting 1 : 1 : 1 crown ether complex and ion-pair formation between the uncomplexed metal ion and the anion in the aqueous phase. The theoretical predictions were verified experimentally by solvent extraction with benzo-18-crown-6 (B18C6), alkali metal (Na, K, Rb) picrates, and various diluents of low dielectric constant. The actual log KMLA values in water at 25 °C of the 1 : 1 : 1 B18C6-alkali metal picrate complexes are 1.54 ± 0.16 for Na, 3.30 ± 0.20 for K, and 3.11 ± 0.24 for Rb. The distribution constants of the 1 : 1 : 1 B18C6 complexes were calculated from the partition constants of B18C6, the formation constants in water of the B18C6-alkali metal-ion complexes, and the actual Kex and KMLA values obtained in this study. The distribution behavior of the 1 : 1 : 1 B18C6 complexes is explained based on regular solution theory. The molar volumes and solubility parameters of the complexes were determined. Moreover, the fundamental equilibria which govern the highest extraction selectivity of B18C6 for K+among the alkali metal ions were completely elucidated.