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

Highlights: ► Determination of arsenic(V) in water of neutral pH. ► An unusual redox couple of elemental Mn/AsV reduces AsV to AsIII. ► Novel manganese coated gold microwire electrode. - Abstract: Direct electrochemical determination of arsenate (AsV) in neutral pH waters is considered impossible due to electro-inactivity of AsV. AsIII on the other hand is readily plated as As0 on a gold electrode and quantified by anodic stripping voltammetry (ASV). We found that the reduction of AsV to AsIII 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 MnII. Advantage is taken of this redox couple to enable for the first time the electrochemical determination of AsV 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 As0 from dissolved AsV caused elemental Mn to be re-oxidised to MnII in a 1:1 molar ratio providing evidence for the reaction mechanism. The deposited AsV is subsequently quantified using an ASV scan. AsIII interferes and should be quantified separately at a more positive deposition potential of −0.9 V. Combined inorganic As is quantified after oxidation of AsIII to AsV using hypochlorite. The microwire electrode was vibrated during the deposition step to improve the sensitivity. The detection limit was 0.2 nM AsV using a deposition time of 180 s.