Direct reduction of As(V) physically attached to a graphite electrode mediated by Fe(III)

As(V) is electrochemically reduced on the surface of a carbonaceous electrode in the presence of hematite and hydrochloric acid 1 M. The influence of other iron oxides (goethite and limonite) was also tested, although they did not provided better results. The potential required to achieve the reduction must be lower than -0.3 V. The anodic voltammograms exhibit a peak at 0.14 V which corresponds to the oxidation of the As degrees previously generated during the pre-treatment step (-1 V, 5 s) to As(III). Fe(II) generated during the pre-treatment step plays a relevant role in the final reduction to As degrees which is subsequently reoxidized to As(III) in the anodic scan. This has been applied to the direct detection of 5 mg kg(-1) of arsenic in a solid sample of compost with high concentration of iron oxides by square wave voltammetry.