Effect of ion complexation on transport selectivity in gold-, and -silver-thiourea-water/nafion membrane systems

Using gold-, and silver-thiourea complexes as model systems, the effects of cornplexation on transport selectivity through Nafion membrane 117 have been investigated with Donnan dialysis in a rotating diffusion cell. In the presence of thiourea (CS(NH2)2)» gold forms only one stable complex (AuT2 +, where T represents the thiourea molecule) while silver forms a number of mono- and polynuclear complexes, with Ag2T6 2+ the dominant species. At low metal concentrations (5x10-4 mol dm-3), the transport rate is found to follow the order Ag+ Au-thiourea species Ag-thiourea species. The transport rates for Ag+ and Au-thiourea species are controlled by boundary layer diffusion while that for the Ag-thiourea species is dominated by membrane diffusion. The transport rate for the Au-thiourea species is about twice as high as that for the silver-thiourea species.