Environ. Sci. Technol. 2004, 38, 2120-2125
Determination of Strong Ligand Sites in Sewage Effluent-Impacted Waters by Competitive Ligand Titration with Silver D. SCOTT SMITH* Department of Chemistry, Wilfrid Laurier University, Waterloo, Ontario, Canada N2L 3C5 RUSSELL A. BELL AND JOHN VALLIANT Department of Chemistry, McMaster University, Hamilton, Ontario, Canada L8S 4M1 JAMES R. KRAMER School of Geography and Geology, McMaster University, Hamilton, Ontario, Canada L8S 4M1
A competitive ligand titration, employing Ag+, is used to determine the binding capacity of the small amounts of strong ligands (SL) in natural water samples. Strong ligands are defined here as high-affinity binding sites for group 11 and 12 metals such as Cu(I), Hg(II), and Ag(I). In addition, the conditional binding strength (log K′) is determined for Ag+ and SL. Diethyldithiocarbamate (DEDC) is the competitive ligand employed. The system is set at constant pH (8.1), ionic strength (0.1 M), and excess-fixed DEDC (10 µM) to determine SLs with log K′ for Ag+ of >10. Silver was chosen as the titrant metal because it binds predominantly with S(-II) versus other ligands and reduced sulfur is thought to comprise the majority of SLs in natural waters. A two-phase system, water and 1,2-dichloroethane (DCE), is required due to the insolubility of Ag-DEDC in water. Added silver partitions into Ag+ and Ag-SL in the aqueous phase and into Ag-DEDC in the DCE phase. An automated system is used to add aliquots of silver and measure Ag-DEDC by UV absorbance in the DCE phase and [Ag+] by specific ion electrode in the aqueous phase. Excess addition of silver and a “Gran’s” analysis gives the binding capacity of SL. The stability constant can also be determined for each addition of silver for an overall onesite SL assumption. Cysteine was used to test the method, and urban waters revealed SL capacities from about 50 to 150 nM and log K′(Ag) of 11-12. An independent analysis of chromium-reducible sulfide correlates well with the SL capacity.
Introduction The evaluation of the speciation of toxic or essential metals, such as Ag(I), Cu(I), Cu(II), Hg(II), Zn(II), Cd(II), and Pb(II), at their low concentrations (
