Determination of strong ligand sites in sewage effluent-impacted waters by competitive ligand titration with silver.
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 microM) 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 one-site 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.
Smith
D Scott
DS
Department of Chemistry, Wilfrid Laurier University, Waterloo, Ontario, Canada N2L 3C5. ssmith@wlu.ca
Bell
Russell A
RA
Valliant
John
J
Kramer
James R
JR
D001665:Binding Sites; D006863:Hydrogen-Ion Concentration; D008024:Ligands; D008962:Models, Theoretical; D012722:Sewage / Q000737:chemistry*; D012834:Silver / Q000737:chemistry*; D017186:Titrimetry; D014873:Water Pollutants / Q000032:analysis*
D008024:Ligands; D012722:Sewage; D014873:Water Pollutants; D012834:Silver