WATER SAMPLE PRESERVATION FOR DISSOLVED ARSENIC(III/V)

Published literature on the time stability of water samples containing dissolved As(III/V) and preserved with a variety of reagents has sparked controversy over the preferred method of preservation. Because the redox state of dissolved arsenic is important to the interpretation of its toxicity, mobility, and geochemical transformations, a proper assessment of its preservation is needed.

Filtering the sample (£0.45 mm), on-site acidification with HCl to a pH<2, and storing chilled in opaque bottles is convenient because it preserves both Fe(II/III) and As(III/V) species in the same bottle; is reliable because arsenic redox species are stable for more than a year under these conditions; and is preferred analytically because few interferences have been encountered when determining Fe(II/III) by colorimetry and when determining As(III/V) by hydride generation – atomic absorption spectrometry. The time stability of As(III) was monitored in 39 surface water and ground water samples from Yellowstone National Park (WY), the Questa Mine Site and Red River Basin (NM), the Summitville Mine Site and Alamosa River Watershed (CO), the Richmond Mine (Iron Mountain, CA), the Penn Mine (CA), Fallon (NV), and Ester Dome (AK). Samples were reanalyzed 2-15 months after the initial determination. The As(T) ranged from 0.006 – 33 mg/L, the initial As(III)/As(T) ratio ranged from 0.02 – 1.0, and 97% of the samples were within 15% of the original determination. The largest differences were for samples with As(III)/As(T) ratios <0.1 where analytical precision is poor because As(III) concentrations approach the method detection limit. These results are consistent with the known chemical properties of dissolved As(III/V). Oxidation-reduction rates of As(III/V) are known to be slow for most natural waters except when Fe(III), reduced sulfur (H₂S, S₂O₂), or iron/arsenic-oxidizing or reducing microorganisms are present in the sample. Filtration removes microorganisms and acidification stabilizes Fe(II/III) so that Fe cannot oxidize and precipitate. Arsenic has never been shown to be photochemically reactive, but oxidation of As(III) in conjunction with the photoreduction of Fe(III) can occur unless light is excluded from during storage. Hence, any method that preserves the Fe redox species will also preserve the As redox species.