A field-deployable instrument for the measurement and speciation of arsenic in potable water

Extensive arsenic contamination of drinking water has been reported in many parts of the world. Trivalent As (As(III)) is regarded to be more toxic than pentavalent As (As(V)) and is slowly oxidized by dissolved oxygen to the latter. Collecting samples in the field and bringing them back to the laboratory is generally unsatisfactory for reliable analysis of As(III) and As(V), due to ongoing oxidative change of the sample. Even simple measurements of total As in the field is problematic due to the inadequate sensitivity of the available colorimetry based field kits. Anodic stripping analysis can be a good field technique to perform total and speciated measurements of dissolved inorganic As down to the 1 mu g/l level. We offer an inexpensive portable stripping voltammetric instrument with a simple procedure for the field analysis of As(III) and As(V). The water sample is acidified to pH 1-2. As(III) and As(V) can be deposited onto a gold film electrode at -0.2 and -1.6 V (vs. Ag/AgCl in 1 M HCl), respectively, then subsequently stripped in a medium of 4.5 M HCl. The As concentrations are determined by a standard addition method to account for any matrix effects. As(III) is first determined at a deposition potential of -0.2 V. Afterwards, all the As is oxidized to As(V) by the addition of a few drops of an oxidant, and total arsenic is determined by a deposition potential of -1.6 V. The original As(V) concentration in the sample is calculated by difference. Influences from foreign ions, organic compounds, pre-electrolysis potential and time and stripping medium have been investigated. The instrument is operated off a desktop or a notebook computer, equipped with an A/D-D/A card. The attainable limit of detection (LOD) is dependent on the deposition time, an LOD of 0.5 mu g/l As(III) or As(V) is obtained for a deposition time of 80 s. For field use. the entire instrument is packaged into a wheeled, padded carrying case (60 x24 x42 cm external dimensions). In addition to a laptop computer, the package weighs 6.5 kg and has an average power consumption of 3.5 W. (C) 1999 Elsevier Science B.V. All rights reserved.