Determination of silver speciation in natural waters. 1. Laboratory tests of Chelex-100 chelating resin as a competing ligand.

Batch and column experiments were performed to investigate the suitability and chemical characteristics of Chelex-100 for use as a competing ligand in ionic silver (Ag(I)) speciation determinations in natural waters. A conditional stability constant (KCcond) for Ag+ chelation by iminodiacetate groups on the surface of Chelex resin was determined by fitting results of batch and column experiments with an equilibrium speciation model. Results of experiments in which Chelex competed with cyanide ion and thiosulfate ion for aqueous Ag+ were fitted well by a model in which log K-cond(Ag-Chelex) was set to 7.2. This value is similar to literature equilibrium constants for a 1:1 AS(+)-EDTA chelate. In batch experiments with Chelex, equilibration times of 24 h were found to be sufficient to bring samples close to equilibrium. Effects of resin counterion and total Ag(I) concentration on extent of Ag(I) chelation were found to be minor. Effect of pH on Ag(I) chelation was minor over a range of 6-10. Column experiments (detention time = 6 s, empty-column basis) in which thiosulfate competed with Chelex for Ag(I) gave similar results to batch experiments with thiosulfate. This implies that batch and column experiments could be compared to explore ligands in natural water systems with different rates of dissociation.