Extraction of Alkali Metal (Na—Cs) Picrates with Dibenzo-18-crown-6 into Various Organic Solvents. Elucidation of Fundamental Equilibria which Govern the Extraction-Ability and -Selectivity

Abstract

In order to quantitatively investigate effects of the size, the structuralrigidity, and the lipophilicity of dibenzo-18-crown-6 (DB18C6) on itsextraction-ability and -selectivity for alkali metal ions, constants of theoverall extraction (Kex), the distribution for various diluents of lowdielectric constants (KD,MLA), and the aqueous ion-pairformation (KMLA) of DB18C6-alkali metal (Na-—Cs) picrate 1:1:1 complexes were determined at 25°C; the partition constants of DB18C6 itself were also measured at 25°C. The log KMLA of Na, K, Rb, and Cs are -0.14 ± 0.11, 1.30 ± 0.10, 1.00 ± 0.09, and 0.24 ± 0.11, respectively. The partition behavior of DB18C6 and its1:1:1 complexes with the alkali metal picrates can be clearly explained byregular solution theory, except for chloroform. The molar volumes andsolubility parameters of DB18C6 and the 1:1:1 complexes were determined.A relation between molar volumes of the complexes and KMLAis discussed. The magnitude of Kex is largely determined by that ofKD,MLA. For every diluent, the extraction selectivity of DB18C6increases in the order Na > Cs > Rb > K. The K extraction-selectivity of DB18C6 over Na is the highest among all the combinations of the two neighboring alkali metals in the periodic table. The extraction-ability and -selectivity for the alkalimetal picrates and their change with the diluent of DB18C6 were completely elucidated by the four fundamental equilibria and regular solution theory.