Hydration Effect on the Ion-pair Extraction of Lithium Picrate by Hydrophobic Benzo-15-crown-5 Ether into Various Less-polar Diluents

The ion-pair formation constant (KMLA0 in mol–1 dm3) for Li(B15C5)+ with a picrate ion (Pic–) in water was determined by potentiometry with a K+-selective electrode at 25˚C and an ionic strength of 0, where B15C5 denotes benzo-15-crown-5 ether. Using the concentration equilibrium constants, KMLA, estimated from this value, the extraction constants (mol–2 dm6 unit) of about ten diluents were re-calculated from previously reported extraction data. Also, the distribution constants of an ion-pair complex, Li(B15C5)Pic, between water and the diluents were re-estimated. A disagreement in the determined KMLA value between a solvent-extraction method and potentiometry was explained in terms of the Scatchard–Hildebrand equation; it came from the fact that the hydration of Li(I) in Li(B15C5)Pic was larger than that of free B15C5 in water. Then, the previously determined value by the former method was re-estimated using the potentiometric KMLA value.