The measurement of silver in road salt by electrothermal atomic absorption spectrometry

The strong complexes formed between silver (Ag+) and chloride (Cl-) may give rise to salt deposits from ancient seas containing elevated Ag concentrations. Since salt from these deposits is frequently used for road de-icing in the northern United States, elevated Ag concentrations in salt represent a potential source of Ag to the areas in which de-icing is performed. In this paper we describe the methodology used to determine the concentration of Ag in salt stored for road application in Madison, Wisconsin, USA. Road salt was dissolved in ultrapure water and analyzed by electrothermal atomic absorption spectrometry. A combination of low Ag concentrations and interferents in the matrix led to pre-concentration and matrix removal via co-precipitation with cobalt pyrrolidinedithiocarbamate and subsequent re-solubilization. Concentrations of Ag in road salt of 102-141 pg g(-1) were determined. Method recovery spikes and blanks indicated that neither analytical interferences nor contamination during sample preparation were significant. Calculations based on these concentrations have led to two important conclusions: (1) based on average ocean salinity values, Ag concentrations in open ocean water and in salt deposits are similar; and (2) compared to other sources, road salt is unlikely to represent an important source of Ag to areas in which salt is used for de-icing.