Extraction of cadmium chloride by 18-crown-6 ether into various diluents: A comparative study with bromide and picrate extraction systems

Abstract

Several key extraction-constants and KD,Cl values were determined at 298K for the extraction of CdCl2 by 18-crown-6 ether (L) into 11 diluents. Their constants were expressed as Kex=[CdLCl2]org/P, Kex±=[CdLCl+]org[Cl−]org/P, KCd/CdL=[CdL2+]org/[Cd2+][L]org with P=[Cd2+][L]org[Cl−]2, and KD,A=[Cl−]org/[Cl−]; the subscript “org” show an organic phase. From these values, other constants were evaluated: K1,org=[CdLCl+]org/[CdL2+]org[Cl−]org {=Kex±/KCd/CdL(KD,Cl)2}, K2,org=[CdLCl2]org/[CdLCl+]org[Cl−]org (=Kex/Kex±), and Kex,ip=[CdLCl2]org/[CdL2+][Cl−]2 (=KexKD,L/KCdL). Here the symbols KD,L and KCdL refer to a distribution constant of L into the org phase and a complex formation constant (mol−1dm3 unit) for Cd2+ with L in water, respectively. Additionally, distribution equilibrium-potential differences (dep) were evaluated from comparing the KD,Cl values with those standardized at dep=0V. In order to clarify reaction properties of CdL2+ and those of CdLA+ in the org phases at A−=Cl−, Br−, and picrate ion (Pic−), magnitudes of K1,org and K2,org were examined and accordingly the change of a coordination structure around Cd(II) was suggested in the reaction of CdLBr+org+Br−org⇌CdLBr2,org. Also, by comparing relative concentrations of CdLA2, CdLA+, and CdL2+ in the org phases, it was clarified that CdLCl+, CdLBr2, and CdLPic+ are dominantly extracted into many phases. Moreover, on the basis of the regular-solution theory plot of log Kex,ip vs. log KD,L, an interaction of water molecules with CdLA2 was discussed.