Voltammetric Characteristics of Hydrous Fe(III) Oxide Embedded into Nafion and Immobilised onto a Screen Printed Carbon Electrode: Binding of Arsenate versus Phosphate

Nafion offers microporous channels of typically 1–4-nm diameter for cation exchange. Recently, it has been shown that these cation exchanger properties can be inverted to allow anion binding by pre-filling the hydrophilic channel structure. Here, pre-filling is performed with hydrous iron oxide and sensitivity towards anionic phosphate and arsenate analytes is investigated. After a period of phosphate/arsenate accumulation, the voltammetric response in aqueous 1 M NaNO3 is obtained based on the Fe(III/II) redox process. The position of the peak at distinct potentials clearly reveals the presence of either phosphate or arsenate, presumably present in the form of FePO4 and FeAsO4. In the presence of mixtures of phosphate and arsenate, a competition of FePO4 versus FeAsO4 nucleation (within the Nafion microporous host) is suggested to result in a switch of phosphate detection at higher concentrations (ca. > 500 μM, solubility controlled) towards arsenate detection at lower concentrations (ca. < 500 μM, nucleation controlled). This phenomenon is suggested to be linked to the Ostwald step rule.