Precious metal extraction by N,N,N′,N′-tetraoctyl-thiodiglycolamide and its comparison with N,N,N′,N′-tetraoctyl-diglycolamide and methylimino-N,N′-dioctylacetamide

A novel tridentate extractant including a soft donor has been developed and examined. The extractant, N,N,N′,N′-tetraoctyl-thiodiglycolamide (TDGA), is analogous in structure to N,N,N′,N′-tetraoctyl-diglycolamide (TODGA) and methylimino-N,N′-dioctylacetamide (MIDOA); however, it contains a sulfur donor instead of the oxygen and nitrogen atoms of TODGA and MIDOA, respectively. The results of the present study show that TDGA can extract silver, palladium, gold, and mercury from acidic solutions to n-dodecane. We investigated the extraction behavior of Ag, Pd, Au, and Hg from four types of acids: HNO3, H2SO4, HCl, and HClO4. We also compared the distribution ratios of hard and soft acid metals using TDGA, TODGA, and MIDOA by investigating the metal complexes with each donor atom. 1H NMR and IR investigations of the metal-TDGA complexes indicated the roles of the donor atoms of TDGA (S and N). The calculated chemical heats of formation of the metal complexes were negatively related to their apparent extraction constants.