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 Original Research Article

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°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, Koex, and ion-pair formation in 1,2-DCE, Koip,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, Kocomp,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 Koex value was discussed. The Kocomp,o value in 1,2-DCE is quite high compared with that in solvating solvents and log Kocomp,o decreases linearly with increasing Gutmann donor number of solvents.