Extraction of sodium and potassium picrates with 15-(2,5-dioxahexyl)-15-methyl-16-crown-5 (lariat 16C5) into various organic solvents. Elucidation of fundamental equilibria which determine the extraction ability for Na+ and K+ and the selectivity

To quantitatively elucidate the effects of the side chains and diluents on the extraction selectivity for sodium and potassium picrates of 15-(2,5-dioxahexyl)-15-methyl-16-crown-5 (L16C5) from the viewpoint of equilibrium, the constants for the overall extraction (Kex), the partition for various diluents of low dielectric constants (KD,MLA), and the aqueous ion-pair formation (KMLA) of L16C5-sodium and -potassium picrate 1:1:1 complexes were determined at 25°C; the distribution constants of L16C5 were also measured at 25°C. The log KMLA values for Na+ and K+ are 2.74±0.29 and 1.70±0.36, respectively. In going from 16-crown-5 (16C5) to L16C5, the side chains decrease the KMLA value, but do not increase the difference in KMLA between Na+ and K+. The distribution behavior of L16C5 and its 1:1:1 complexes with the alkali metal picrates closely obeys regular solution theory, except for chloroform. Molar volumes and solubility parameters of L16C5 and the 1:1:1 complexes were determined. The magnitude of Kex is mainly governed by the KM(L16C5)A value. For every diluent, L16C5 shows Na+ extraction selectivity over K+. The Na+ extraction selectivity of L16C5 is determined completely by KM(L16C5)A. The extraction ability and selectivity for sodium and potassium picrates by L16C5 are compared with those of 16C5 on the basis of the fundamental equilibrium constants.