Solvent Extraction of Uni- and Bivalent Metal Picrates with 16-Crown-5 into Benzene

Abstract

Abstract Distribution constant (KD,L) of 16-crown-5 (16C5) and overall extraction equilibrium constants (Kex) of uni- and bivalent metal picrates with 16C5 were determined between benzene and water at 25 °C. The Kex values were analyzed by four fundamental equilibria, extraction constant of picric acid, KD,L formation constant in water and ion-pair extraction constant (Kex,ip) with a picrate ion of the 16C5–metal ion complex. The Kex value of 16C5 for the uni- and bivalent metal ions decreases in the order Ag+>Na+>Tl+>K+>Rb+>Cs+>Li+ and Pb2+>Ba2+≥Sr2+>Ca2+, respectively. Size-fit relationship holds for extraction-selectivity of 16C5 toward alkali metal ions, but it does not hold for that toward alkaline earth metal ions. Factors influencing extractability and extraction-selectivity of 16C5 for metal ions were discussed from the viewpoint of molecular grounds. Kex,ip value of 16C5 varies with a cation of the same charge held in the cavity. Kex,ip values of 16C5–metal picrate complexes were compared with those of 15-crown-5–metal picrate complexes. Various effects on ion-pair extractability of a 16C5–metal ion complex with a picrate anion were explained.