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 +

Abstract

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 ( K ex) and aqueous ion-pair formation ( K MLA) of B18C6-sodium and potassium perchlorate 1:1:1 complexes (MLA) were determined at 25 °C. The K ex value was analyzed by the four fundamental equilibrium constants. The K MLA values were determined by applying our established method to this perchlorate extraction system. The K M(B18C6)A value of the perchlorate is much larger for K + than for Na +, and is much smaller than that of the picrate. The K M(B18C6)A value makes a minor contribution to the magnitude of K ex 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 KClO 4 over NaClO 4, which is determined mostly by the much greater log/(log K MLA) 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 K K(B18C6)A)−log/(log K Na(B18C6)A) value for the perchlorate than for the picrate. The perchlorate system is recommended for extraction separation of K + from Na +.