Abstract

Three kinds of extraction constants, K_ex, K_ex±, and K_ex+, 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===MLA_2,Bz, and A^-===A^-_Bz. Here, K_ex, K_ex±, and K_ex+ 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 K_ex 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.

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