Trace iron determination by cyclic and multiple square-wave voltammetry atnafion coated electrodes. Application to pore-water analysis

Nafion coated glassy carbon electrodes (NCE) are employed for preconcentrat ing and detecting Fe2+ and Fe3+ cations from aqueous solutions. The influen ce of the supporting electrolyte composition and of the redox state on the analyte partitioning within the Nafion coating are examined. By using cycli c voltammetry, the ion-exchange voltammetric determination of iron in the]I M concentration range is achieved from voltammetric peak currents relevant to the reversible redox process: Fe3++e = Fe2+. Depending on the starting p otential of the voltammetric scan, all the iron is initially convened to th e Fe(III) or Fe(II) redox state, so that, from the voltammogram, the overal l concentration of iron is always obtained However, the simple measurement of the open circuit potential at the NCE before starting the scan allows on e to get information on the ratio between Fe(III) and Fe(II) incorporated i n the Nafion coating. From relevant ion-exchange distribution coefficients, open circuit potential values can be related to the Fr(III)/Fe(II) concent ration ratio in the sample. The use at the NCE of a new electroanalytical t echnique named multiple square-wave voltammetry in the double differential mode allows the significative lowering of detection limits pushing the oper ative range of the method in the nM range. Application to analysis of Fe(II ) in the pore-waters of sediments of the lagoon of Venice (Italy) is presen ted.