Voltammetric Arsenic Speciation

A square-wave anodic stripping voltammetric method was developed for the determination of total arsenic as well as speciation of As(III) and As(V) in weakly acidified (pH 1-2) water and seawater using 25 µm gold microwire electrodes. Existing methods suffer from instability of the As(III) peak and hydrogen formation under strongly acidic conditions. It is demonstrated, that chlorine, generated at the auxiliary electrode during the deposition step from the oxidation of chloride causes oxidation of dissolved As(III) to As(V) and thus a gradual decrease of the As(III) peak. This effect is minimised by lowering the reduction current during the deposition step, for instance by using a microwire electrode, increasing the pH to 2, using a more positive deposition potential (Edep), removing dissolved oxygen, increasing the cell volume, or by using a flow cell. As(III) is detected selectively in pH 2 water at Edep=-0.4 V. As(V) is detected after further acidification to pH 1 and deposition at -1.4 < Edep < -1 V. Optimised conditions for the chemical speciation of As(III) and As(V) in seawater, tap water and mineral water included acidification with 0.1 M HCl, a deposition potential of -0.4 V for As(III) and of -1.4V for total As. The new procedure and microwire electrode give a considerably lower limit of detection (1.5 nM As for 30 s deposition and 0.06 nM As after 15 min deposition) than previous voltammetric methods and a stable signal. Similar results were obtained using stripping chronopotentiometry.