KINETICS AND MECHANISM OF BACK-EXTRACTION - SELECTED PALLADIUM EXTRACTION SYSTEMS

The equilibrium and kinetics of back-extraction (stripping) of palladium originally extracted as PdCl4(2-) from the chloroform extracts obtained with 1-(2-pyridylazo)-2-naphthol (PAN) chloroform extracts obtained with 1-(2-pyridylazo)-2-naphthol (PAN), 7-(4-ethyl-1-methyloctyl)quinolin-8-ol (Kelex 100) or diocyte sulfide (R2S) were investigated. Replacement of chloride in extracted species by thiocyanate occurs prior to back-extraction. The back-extraction equilibria have been described by Pd(SCN)2(R2S)2(o) + [GRAPHICS] Pd(SCN)4(2-) + PAN(o), with K(BX2) = 10(4.89 +/- 0.06). The rate of stripping from Pd-PAN and Pd-Kelex 100 displayed an inverse first-order dependence on the solution pH, a second-order dependence on the thiocyanate concentration and was zero order in both the chloride and the organic phase chelate concentration. More complicated kinetics were observed for palladium stripping in the dioctyl sulfide system. In all systems, the enhancement in stripping rate parallels the size of the "trans effect".