Abstract
Abstract Overall extraction equilibrium constants (Kex) of various metal picrates (MAm) with benzo-18-crown-6 (B18C6) and distribution constant (KD,L) of B18C6 were determined at 25 °C between water and chloroform. The Kex values were analyzed by constituent equilibria, e.g., KD,L, formation constant in water and ion-pair extraction constant (Kex,ip) with a picrate anion of the B18C6–metal ion complex. Kex orders for univalent and bivalent metal ions are Tl+>K+>Rb+>Ag+>Cs+>Na+ and Pb2+>>Sr2+>Ca2+, respectively. Extraction selectivities of B18C6 for the alkali and alkaline earth metal ions are consistent with the size-fit concept; namely, the more closely a cation fits into a crown ether cavity, the more extractable the complex is. For a given B18C6–Mm+ system, Kex,ip value is much larger than values of the other constituent equilibria. Thus, the magnitude of Kex value is determined largely by that of Kex,ip value. Kex,ip value of B18C6 for univalent and bivalent metal ions increases in the order Na+<Ag+<Cs+<K+<Rb+<Tl+ and Sr2+<Ca2+<Pb2+, respectively. Factors governing the magnitude of the Kex,ip value were discussed in detail.
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