Stability of inorganic arsenic (III) and arsenic (V) in water samples

The objective of this study was to formulate a protocol for the collection and preservation of natural water samples (lakes and streams) to be analysed for the inorganic species As(III) and As(V). The analytical technique employed was HPLC-ICP-MS, using a proprietary anion exchange resin (ANX-1606-AS from Cetac) as the basis for separation of the two species. Initial experiments carried out at room temperature, where de-ionised water and Ottawa River water samples were spiked at low (0.5-20) µg l –1 concentrations of As(III) and As(V), demonstrated that As(V) is actually reduced to As(III) within a few days. This reduction is matrix and concentration dependent and was confirmed by independent analysis using HGAAS with hydride generation at pH 4.5 and <1. Such conversion of As(V) to As(III) does not occur in spiked deionised water maintained at 5 °C but is evident in the Ottawa River sample at this temperature. A time and temperature study of a filtered (0.45 µm) natural water sample, with a total As concentration of 21 µg l –1 , showed that immediate storage in a filled bottle at about 5 °C will preserve As (III) and As(V) concentrations for about 30 d. If kept at room temperature, changes for this particular sample occur after about 5 d, with As(V) becoming the dominant species via oxidation and gradually declining thereafter. Although acidification to 0.1% HNO 3 stabilises the As species for at least 15 d at 22 °C, its effect is immediately to alter the species distribution. With the natural water sample, this effect was to increase the concentration of As(III) substantially and, to a lesser extent, that of As(V), at the expense of other forms of the element. Acidification to 0.1% HCl also produced these results. The study of spiked de-ionised water and Ottawa River samples at 0.1 and 0.4% in HNO 3 and HCl demonstrated that both acids cause the oxidation of As(III) to As(V) but HNO 3 showed a higher degree of oxidation with greater acid strength, as expected.

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