Extraction of Several Divalent Metal Picrates by 18-Crown-6 Ether Derivatives into Benzene: A Refinement of Methods for Analyzing Extraction Equilibria

Abstract

Three kinds of extraction constants, Kex, Kex±, and Kex+, were evaluated from the improved model that the following three component-equilibria were added to a previously-proposed model for an overall extraction: M2+ + A-===MA+, MLA+Bz + A-Bz===MLA2,Bz, and A-===A-Bz. Here, Kex, Kex±, and Kex+ were defined as [MLA2]Bz/([M2+][L]Bz[A-]2), [MLA+]Bz[A-]Bz/([M2+][L]Bz[A-]2), and [MLA+]Bz/([M2+][L]Bz[A-]), respectively; the subscript “Bz” denotes benzene as an organic phase. The symbols correspond to M2+ = Ca2+ and Pb2+, L = 18-crown-6 ether (18C6) and dibenzo-18C6 (DB18C6), and A- = picrate ion. The ion-pair formation constant for M2+ + A-===MA+ at M2+ = Pb2+ in an aqueous phase was also determined at 298 K and ionic strength of zero by an extraction of HA into 1,2-dichloroethane and Bz with the presence of Pb2+ in the aqueous phase. The Kex values re-evaluated from the present model were in agreement with those determined by the previous extraction model. Individual distribution constants of A- into Bz were almost constant irrespective of kinds of M2+ and L employed. Furthermore, the composition-determination method of the ion pairs, MLA2, extracted into Bz was re-examined. Similar analyses were performed in the SrA2-, BaA2-18C6, and SrA2-DB18C6 systems without considering the formation of MA+ in the aqueous phases.