Determination of arsenate in natural pH seawater using a manganese-coated gold microwire electrode.

Direct electrochemical determination of arsenate (As(V)) in neutral pH waters is considered impossible due to electro-inactivity of As(V). As(III) on the other hand is readily plated as As(0) on a gold electrode and quantified by anodic stripping voltammetry (ASV). We found that the reduction of As(V) to As(III) was mediated by elemental Mn on the electrode surface in a novel redox couple in which 2 electrons are exchanged causing the Mn to be oxidised to Mn(II). Advantage is taken of this redox couple to enable for the first time the electrochemical determination of As(V) in natural waters of neutral pH including seawater by ASV using a manganese-coated gold microwire electrode. Thereto Mn is added to excess (~1 μM Mn) to the water leading to a Mn coating during the deposition of As on the electrode at a deposition potential of -1.3 V. Deposition of As(0) from dissolved As(V) caused elemental Mn to be re-oxidised to Mn(II) in a 1:1 molar ratio providing evidence for the reaction mechanism. The deposited As(V) is subsequently quantified using an ASV scan. As(III) interferes and should be quantified separately at a more positive deposition potential of -0.9 V. Combined inorganic As is quantified after oxidation of As(III) to As(V) using hypochlorite. The microwire electrode was vibrated during the deposition step to improve the sensitivity. The detection limit was 0.2 nM As(V) using a deposition time of 180 s.