Preservation of water samples for arsenic speciation analysis is a tricky issue. While adding oxidizing acids such as nitric acid is a common approach for stabilizing samples for trace element determination, such approach is not feasible for speciation analysis, where the redox-conditions should not be changed in order to avoid any species transformation. The research group at the UFZ headed by R. Wennrich now found a solution to stabilize water sanples for up to three months.
The influencing factorsArsenic is well-known to exist in two oxidation states (arsenite and arsenate) in water samples which can easily change. With regard to preservation of the species in their original state when sampling takes place, the influencing factors of the two arsenic species on the redox-reaction have to be discussed. The following points have to be considered:
- redox potential,
- redox couples,
- precipitation/sorption,
- microbial activity,
- light, and
- temperature.
While theoretically, the redox potential together with the pH value determines the ratio between the two arsenic species, in reality arsenic species are often not in equilibrium due to kinetic effects.
Preservation StrategiesAnother strategy for preservation consists of the removal or complexation of potential redox counterparts without influencing the equilibrium between the different arsenic species. Since oxygen alone, originating from air, is not able to oxidize arsenite, an additional redox counterpart, such as dissolved or colloidal iron or manganese compounds seems to be necessary to mediate the reactions. Consequently, the exclusion of these counterparts from the reaction could be a possible method for successful preservation by means of complexation. However, due to the possibility of co-porecipitation, a prevention of all precipitation processes is imperative for species preservation.
Light and temperature are additional factors, which have to be considered. The influence of light on the oxidation of arsenite is important and therefore keeping the water samples in the dark is advisable. Temperature has influence on microbial activity as well as on chemical reactions (endothermic/exothermic) and therefore cooling the water samples is also advisable.
The
analytical research group at the UFZ headed by
Reiner Wennrich found that the pH should be 2 or less in order to avoid precipitation with respect to iron phosphates. Using 10 mM H
3PO
4 for preservation water samples were stable with respect to arsenic speciation for 28 days in the presence of high concentrations of both iron and manganese (up to 100 mg L-1). The concentration of arsenic species in all samples was higher than 90% of the initial value after 3 months, with the tendency towards a slight diminution of arsenite.
The UFZ study
Birgit Daus, H. Weiss,
Jürgen Mattusch,
Rainer Wennrich,
Preservation of arsenic species in water samples using phosphoric acid - Limitations and long-term stability, Talanta, 69/2 (2006) 430-434.
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