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

Ion-pair formation constants (KMLA°) 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 KMLA 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 KMLA 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 KMLA. 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.