Extraction equilibria of various metal picrates with 19-crown-6 between benzene and water. Effect of the extra methylene group on extraction ability and selectivity

The overall extraction equilibrium constants (K-ex) of picrates of Li+, Na, K+, Rb+, Cs+, Ag+, Tl+, and Sr2+ with 19-crown-6 (19C6) were determined b etween benzene and water at 25 degrees C. The K-ex values were analyzed int o the constituent equilibrium constants, i.e. the extraction constant of pi cric acid, the distribution constant of the crown ether, the formation cons tant of the metal ion-crown ether complex in water, and the ion-pair extrac tion constant of the complex cation with the picrate anion. The effects of an extra methylene group of 19C6 on the extraction ability and selectivity are discussed in detail by comparing the constituent equilibrium constants of 19C6 with those of 18-crown-6 (18C6). The K-ex value of 19C6 for each me tal ion is lower than that of 18C6, which is mostly attributed to the highe r lipophilicity of 19C6. The extraction ability of 19C6 for the univalent m etal ions decreases in the order Tl+ > K+ > Rb+ > Ag+ > Cs+ > Na+ much grea ter than Li+, which is the same as that observed for 18C6. The difference i n log K-ex, between the univalent metals is generally smaller for 19C6 than for 18C6. The extraction selectivity of 19C6 is governed by the selectivit y in the ion-pair extraction, whereas that of 18C6 depends on both the sele ctivities in the ion-pair extraction and in the complexation in water. (C) 1999 Elsevier Science B.V. All rights reserved.