Effect of a low concentration of organic solvent in the aqueous phase on ion-pair solid-phase extraction

Abstract

It was found that a low concentration of organic solvent dissolved in the aqueous phase remarkably enhances the ion-pair extraction onto a solid-phase. The ion-pair distribution between solid and aqueous phases was studied in the presence of organic solvent dissolved in the aqueous phase. The magnitude of the extraction was expressed by the extraction constant (K_ex) defined as: K_ex=[C⁺·A^-]_s/{[C⁺][A^-]} where C⁺ and A^- denote the cation and anion studied, and the subscript s expresses the solid phase. Bis [2-(5-chloro-2-pyridylazo)-5-diethylaminophenolato]cobalt(III) ion and dodecanesulfonate ion were the cation and anion studied. A cellulose nitrate membrane filter and octadecyl modified silica gel (ODS) were studied as the solid phase. Dissolved organic solvents, nitrobenzene, o-xylene and benzene at 28×10^-3 mol dm^-3 in the aqueous phase, enhanced the distribution of the ion pair onto the solid-phases. In particular, in the case of ODS, o-xylene and nitrobenzene gave 2.5 and 2.2 greater log K_ex values, respectively, compared with those in the absence of the solvents. The drastic enhancement of extracting power onto the solid-phase by the dissolved organic solvent will be useful for the improvement of collection efficiency in preconcentration for trace analysis.