Role of SCN^- in the liquid-liquid extraction of Pd(II) by Kelex 100 in Toluene from aqueous chloride solutions. The equilibrium approach

The liquid-liquid extraction of Pd(II) by 7-(4-ethyl-1-methyloctyl)quinolin-8-ol[Kelex 100 (HL)] in toluene from 1 M aqueous chloride solutions in the presence of SCN- is described. The investigations included both elucidation of the equilibrium extraction reactions by measurement of the metal distribution and the characterization of extraction kinetics by measuring the Pd(II) extraction rate. In both studies the experimental data were interpreted by chemical models obtained with appropriate numerical calculations by using the programs LETAGROP-DISTR and KILET. In the case of extraction equilibria, it is suggested that the complex PdL2 and PdSCNL are formed in the organic phase and PdClL in the aqueous phase to explain the metal distribution observed experimentally. With respect to extraction kinetics, the "best" model was found taking into account the reversible reaction to the formation of aqueous Pd-SCN--Cl- species with the organic extractant Kelex 100. The results obtained for both equilibrium and kinetic data reveal that SCN- behaves as a catalytic reagent, in agreement with previous results. © 1993.