Extraction of sodium and potassium perchlorates with benzo-18-crown-6 into various organic solvents. Quantitative elucidation of anion effects on the extraction-ability and -selectivity for Na+ and K+

To investigate quantitatively the anion effect on the extraction-ability and -selectivity of benzo-18-crown-6 (B18C6) for alkali metal ions, the constants for overall extraction into various diluents having low dielectric constants (Kex) and aqueous ion-pair formation (KMLA) of B18C6-sodium and potassium perchlorate 1:1:1 complexes (MLA) were determined at 25 °C. The Kex value was analyzed by the four fundamental equilibrium constants. The KMLA values were determined by applying our established method to this perchlorate extraction system. The KM(B18C6)A value of the perchlorate is much larger for K+ than for Na+, and is much smaller than that of the picrate. The KM(B18C6)A value makes a minor contribution to the magnitude of Kex for the perchlorate system, but a major contribution to that for the picrate one. The distribution behavior of the B18C6 1:1:1 complexes with the alkali metal perchlorates follows the regular solution theory. For the diluent with a high dipole moment, however, the 1:1:1 complexes somewhat undergo the dipole–dipole interaction. B18C6 always shows very high extraction selectivity for KClO4 over NaClO4, which is determined mostly by the much greater log/(log KMLA) value for K+ than for Na+. The extraction-ability and -selectivity of B18C6 for Na+ and K+ ions with a perchlorate ion were compared with those with a picrate ion in terms of the fundamental equilibrium constants. The K+ extraction-selectivity of B18C6 over Na+ for the perchlorate system is superior to that for the picrate one, which is caused largely by the greater log/(log KK(B18C6)A)−log/(log KNa(B18C6)A) value for the perchlorate than for the picrate. The perchlorate system is recommended for extraction separation of K+ from Na+.