Speciation of arsenic in paddy field water by square wave cathodic stripping voltammetry using a hanging drop mercury electrode.

Arsenic is found widely in nature, with mining and the use of pesticides in agriculture being the leading carriers of this metalloid. Considering the harm caused by As in cultivation media, a simple and sensitive method for arsenic speciation in paddy field water samples was developed by square wave cathodic stripping voltammetry using a hanging mercury drop electrode. Due to the low solubility of arsenic in mercury, this element was combined with Cu to form an intermetallic compound that is highly soluble (CuxAsy). In the case of As(V), it must first be reduced to As(III) with sodium thiosulfate to be later quantified by the difference in concentration between the inorganic As and As(III) total. Parameters such as concentration of HCl, Cu, and sodium thiosulfate (only for As(V)), potential deposition, frequency, and amplitude were evaluated. Under ideal conditions, limits of detection of 0.025 and 0.12 μg L-1 and limits of quantification of 0.082 and 0.39 µg L-1 were obtained for As(III) and As(V), respectively. The method presented high accuracy (recoveries from 97 to 110%) and was applied to determine both species in ten samples of paddy field water.