Ion-pair formation constants (K MLX in mol-1·dm3) for M(B18C6)+, M(15C5)+, or M(B15C5)+ with pairing anions (X-) in water were determined by potentiometry with ion-selective electrodes at 25 °C over wide ranges of the ionic strength (I). The symbols B18C6, 15C5, and B15C5 denote benzo-18-crown-6 ether, 15-crown-5 ether, and benzo-15C5, respectively; metal salts, MX, used are sodium picrate (NaPic), KPic, NaBPh4, NaMnO4, and KMnO4. The K MLX values at I = 0 mol dm-3 (K MLX°) were evaluated from analyzing the I dependence of the log K MLX values determined. Then, effects of shapes of X-, the benzo group of the ethers, their ring sizes, and the shielding of the metal ions, M+, by the ethers on these K MLX° values were discussed in comparison with the values previously reported on M(18C6)X, M(15C5)Pic, and M(B15C5)Pic. Also, center-to-center distances in the M(18C6 derivatives)+-X- or M(15C5 ones)+-X- pairs were estimated from Bjerrum's equation.

No abstract

No abstract

Ion-pair formation constants (KMLA) for alkali metal-crown ether complex ions (ML + ) with a pairing anion (A -) in water have been determined by applying the regular solution theory for solventextraction systems with these species and less-polar diluents, and also discussed for solute-solute and -solvent interactions in the systems. 1,2 In following these studies, a potentiometric method for the KMLA determination has also been developed and established. 3,4 Depending on kinds of crown ethers (L) used, however, the KMLA values determined by both of these methods either agreed well or did not. Regarding this disagreement, the authors clarified its cause, and then found out the fact that the disagreement is due to a distribution property of L. 4 Consequently, the extraction systems have been classified in two groups, corresponding to the distribution properties. 4 Namely, both the KMLA values agreed well for systems with a hydrophobic L, such as benzo-15-crown-5 ether (B15C5) and benzo-18-crown-6 one (B18C6), while those did not with a hydrophilic L, such as 15-crown-5 ether (15C5) and 18-crown-6 one (18C6). Using the Scatchard-Hildebrand equation, 5 these facts could be explained in terms of the magnitude of an interaction of L or the ion-pair complex MLA with a water molecule in a water (w) phase. 4 It was also found that the interaction of the Li(I) ion-pair complex, Li(18C6)Pic, with a water molecule was comparable, in magnitude, to that of the hydrophilic 18C6 with a water molecule, 4 where Pic -denotes a picrate ion. This finding suggests a possibility that Li(I) held in ion-pair complexes with the hydrophobic L more-strongly interacts with water molecules than does the free L. Thus, the KMLA 0 value for a LiPic-B15C5 system was determined potentiometrically at 25˚C and an ionic strength (I) of 0, and consequently the KMLA disagreement was observed here.In this study, based on this system, we wish to report another limitation causing the disagreement in KMLA between the two methods, and discuss its cause. Also, the extraction constants (Kex) of about ten diluents were re-calculated, using this KMLA 0 value and corresponding extraction data 2 reported before by the co-authors. Furthermore, the distribution constants (KD,MLA) of the ion-pair complex, Li(B15C5)Pic, between the water phase and the diluents were re-estimated. In addition to this, the KMLA value (KMLA ext. one) determined previously by the extraction method 2 was re-estimated, based on the potentiometric KMLA 0 value. Experimental ChemicalsLithium picrate and B15C5 were prepared or purified by the same methods as those 4 described previously. The concentrations of aqueous LiCl solutions were determined by precipitation titration with a standardized AgNO3 solution. Other reagents were of analytical grade and used without further purification. Water that was used to prepare all aqueous solutions was purified by passing through a Milli-Q Labo ultrapure system. Emf measurementsEmf values were measured at 25 ± 0.4˚C by using the following cell: Ag | A...