Fundamental equilibrium analysis on the ion-pair extraction of dibenzo-18-crown-6 complex of alkali metal ions with picrate ion into 1,2-dichloroethane

The ion-pair extraction equilibria of dibenzo-18-crown-6 (DB18C6) complexes of Na+, K+, Rb+ and Cs+ (M+) with picrate ion (Pic(-)) into 1,2-dichloroethane (1,2-DCE) have been studied at 25.0 degrees C. In the case of the Rb+ and Cs+ systems, the extraction results were interpreted by taking into consideration the formation of a (DB18C6)(2) -M+ complex in 1,2-DCE. The thermodynamic constants of extraction, K-ex(o), and ion-pair formation in 1,2-DCE, K-ip,o(o) of ion pairs of the DB18C6-M+ complexes with Pic(-) were determined. By using the distribution coefficient of M+.Pic(-) the thermodynamic formation constants of the DB18C6-M+ complexes in 1,2-DCE, K-comp,o(o), were evaluated. Consequently the component equilibrium constants of the ion-pair extraction were completely determined and a contribution of these constants to the difference of K-ex(o) value was discussed. The K-comp,o(o) value in 1,2-DCE is quite high compared with that in solvating solvents and log K-comp,o(o) decreases linearly with increasing Gutmann donor number of solvents. (C) 1998 Elsevier Science B.V. All rights reserved.