The stability constants of the aqueous chloro complexes of divalent zinc, cadmium and mercury determined by solvent extraction with tri-n-octylphosphine oxide

Abstract

The extraction of divalent zinc, cadmium and mercury from aqueous solutions containing lithium chloride using tri-n-octylphosphine oxide (TOPO) in benzene has been investigated. By applying the trial and error method of least-squares analysis to the equilibrium equation, M 2+ (aq) + 2Cl − (aq) + 2TOPO (org) ⇆ MCl 2 · 2TOPO (org), for these extraction systems, the stability constants of the aqueous chloro complexes of divalent zinc, cadmium and mercury have been determined as follows: for zinc (II) β 1 = 5.40, β 2 = 0.800, β 3 = 0.300 and β 4 = 0.102, and K = 3 × 10 4; for cadmium (II) β 1 = 320, β 2 = 210, β 3 = 200 and β 4 = 85.0, and K = 3.5 × 10 4; for mercury (II) β 1/ K = 1.60 × 10 −8, β 2/ K = 1.20 × 10 −4, β 3/ K = 1.54 × 10 −3 and β 4 K = 1.53 × 10 −3. Furthermore, distribution of the species of these metals which exist in aqueous chloride solutions has been calculated by using the values of the stability constants obtained.