Quantitative extraction separation systems of alkali and alkaline earth metal ions using cryptands as ion-size selective masking reagents

Abstract

Quantitative separation systems have been successfully developed in the synergetic extraction of alkali and alkaline earth metal ions with benzoyltrifluoroacetone (BFA) and tri- n-butylphosphate (TBP) or tri- n-octylphosphine oxide (TOPO) into cyclohexane in the presence of cryptand as an ion-size selective masking reagent. Diazapolyoxabicyclic ligands (cryptands) exhibit the possibility to separate alkali and alkaline earth metal ions. On adding cryptand to the aqueous phase, the extractions of metal ions with larger ionic radii were markedly reduced. Consequently, the separation of the metal ions was greatly enhanced. Since the masking effect by cryptands depends on the pH, it is desirable to select the extractants which have the pKa values in alkaline region. Cryptand[2.2.1] and [2.2.2] were selected as the masking reagents in order to get the quantitative separations of alkali or alkaline earth metal ions. Quantitative extraction separations between Mg and Ca, and Li and Na were achieved in the synergetic extraction with BFA and TBP or TOPO in the presence of cryptand[2.2.1]. Ca could also be quantitatively separated from Sr with BFA and TBP in the presence of cryptand[2.2.2]. The stability constants of alkali and alkaline earths with cryptand[2.2.1] and [2.2.2] were obtained.