Synergistic effect of tri-n-octylphosphine oxide upon extractive separation of zinc(II) and cadmium(II) by 5-(4-nitrophenylazo)-7-(4-ethyl-1-methyloctyl)-8-quinolinol

The synergistic extraction of zinc(II) and cadmium(II) with 5-(4-nitrophenylazo)-7-(4-ethyl-1-methyloctyl)-8-quinolinol (HNEQ) into 1,2-dichloroethane (DCE), chloroform and 1-octanol in both the absence and presence of tri-n-octylphosphine oxide (TOPO) as well as the mutual separation of these metal ions were investigated. Various equilibrium constants were determined. Upon the addition of TOPO on the Zn (II)-HNEQ and Cd(II)-HNEQ systems, the extraction percentage-pH curves shifted toward a more acidic region. The synergistic extraction constant for the Zn(II)-HNEQ-TOPO-DCE system, K-ex,K-s (= [Zn(NEQ)(2)(TOPO)][H+](2)[Zn(II)](-1)[HNEQ](-2)[TOPO](-1)), was determined to be 10(-0.43) at 25 degreesC and I = 0.1 M (H, Na) ClO4. The formation constant, beta (a,I) (= [Zn(NEQ)(2)(TOPO)](org)[Zn(NEQ)(2)](-1)(org)[TOPO](-1)(org)), of the association complex in DCE was found to be 10(5.65) at 25 degreesC. The addition of TOPO to Zn(II)-HNEQ-DCE system showed a larger synergistic effect than that to the Zn(II)-HNEQ-chloroform system. The separation efficiency of zinc(II) and cadmium(II) was enhanced upon the addition of TOPO and chloride ion using DCE as a solvent.