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 analyse d for the inorganic species As(In) and As(V). The analytical technique empl oyed 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 experi ments carried out at room temperature, where de-ionised water and Ottawa Ri ver water samples were spiked at low (0.5-20) mu g l(-1) concentrations of As(III) and As(V), demonstrated that As(V) is actually reduced to As(III) w ithin a few days. This reduction is matrix and concentration dependent and was confirmed by independent analysis using HGAAS with hydride generation a t pH 4.5 and <1. Such conversion of As(V) to As(In) does not occur in spike d deionised water maintained at 5 degrees C but is evident in the Ottawa Ri ver sample at this temperature. A time and temperature study of a filtered (0.45 mu m) natural water sample, with a total As concentration of 21 mu g l(-1), showed that immediate storage in a filled bottle at about 5 degrees C will preserve As (III) and As(V) concentrations for about 30 d. If kept a t room temperature, changes for this particular sample occur after about 5 d, with As(V) becoming the dominant species via oxidation and gradually dec lining thereafter. Although acidification to 0.1% HNO3 stabilises the As sp ecies for at least 15 d at 22 degrees C, its effect is immediately to alter the species distribution. With the natural water sample, this effect was t o increase the concentration of As (In) substantially and, to a lesser exte nt, that of As(v), at the expense of other forms of the element. Acidificat ion 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 HNO3 and HCl demonstrate d that both acids cause the oxidation of As(III) to As(V) but HNO3 showed a higher degree of oxidation with greater acid strength, as expected.