Preservation Of Water Samples For Arsenic(Iii/V)Determinations: An Evaluation Of The Literature And New Analytical Results
McCleskey, R. Blaine 1 ; Nordstrom, D. Kirk 2 ; Maest, Ann S. 3
1 U.S. Geological Survey, Boulder, CO
2 U.S. Geological Survey, Boulder, CO
3 Buka Environmental, Boulder, CO
Published literature on the time stability of water samples containing dissolved As(III/V) and preserved with a variety of reagents has shown contradictory results (1-3). The redox state of dissolved As is important to the interpretation of its toxicity, mobility, and geochemical transformations. Aqueous As redox species are affected by both biotic (4) and abiotic processes (5). Photo-catalyzed Fe(III) reduction can cause As(III) oxidation (6; fig. 1a). However, storing the sample in the dark prevents photochemical reactions and the presence of Fe(II) or SO4inhibits (fig. 1b) the oxidation of As(III) by Fe(III). In our experience, completely oxidized Fe does not coexist with completely reduced As in natural water samples. Furthermore, natural water samples containing high concentrations of Fe(III) will contain either Fe(II), SO4, or both. Consequently, As(III) oxidation reactions observed in the laboratory are not observed in natural samples. Figure 1c shows 5 water samples, preserved with HCl, that have a range of pH, Fe(II/III), As(III/V) and SO4 concentrations that were monitored for their As(III/V) ratio for 5-19 months. No significant change in As(III) concentration could be detected in these samples. Any field collection procedure that filters out microorganisms, adds a reagent that prevents dissolved Fe and Mn oxidation and precipitation, and isolates the sample from solar radiation will preserve the As(III/V) ratio.
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