On the difference between ion-pair formation constants of crown ether-complex ions with picrate ion in water determined by solvent extraction and by potentiometry

Abstract

Ion-pair formation constants ( K MLA°) for crown ether-complex ions (ML +) with picrate ion (Pic −) in water (w) at the ionic strength ( I) = 0 mol dm − 3 and 25 °C were determined by potentiometry with ion-selective electrodes for MPic-15-crown-5 ether (15C5), -benzo-15C5 (B15C5) (M = Na, K) and LiPic-18-crown-6 ether (18C6) systems. Also, the same kind of constant for Li + with Pic − was determined potentiometrically. The K MLA values estimated at given I were compared with those given previously by solvent extraction with its data analysis by the regular solution theory. Consequently, a large difference between these K MLA values was observed for the 15C5 systems; similar differences had been obtained for NaPic- and KPic-18C6 ones. The cause for these differences was discussed based on the Scatchard-Hildebrand equation and then the differences were corrected by referring to the potentiometric K MLA. In the correction, extraction constants for the above systems into more than ten organic (o) solvents were recalculated together with distribution constants for the ion-pair complexes ML +Pic − between the w- and o-phases.