Iron(II) and iron(III) determination by potentiometry and ion-exchange voltammetry at ionomer-coated electrodes

This paper examines the possibility to combine potentiometric and voltammetric measurements, both performed at ionomer coated glassy carbon electrodes, in order to determine the concentration and gain information on the redox state of electroactive ions even when they are present at trace (submicromolar) concentration levels. As a model case, the speciation of iron(H) and iron(III) is studied both for the cations Fe2+ and Fe3+ and for the anions Fe(CN)(6)(4-) and Fe(CN)(6)(3-). For the first situation, electrodes coated with Nation are employed while for the latter electrodes coated with the anion exchanger Tosflex are examined. The ratios between oxidised and reduced species are measured at trace levels thanks to the ion-exchange preconcentration capabilities of the coatings. Relevant equations ruling the potentiometric responses at the coated electrodes have been derived by proper adjustment of the Nernst equation. Ion-exchange voltammetry at the coated electrodes allowed the measurement of parameters, such as ion-exchange distribution coefficients, which are relevant for the experimental testing of the suitability of these equations. Voltammetry at the coated electrodes allowed also the sensitive determination of the total concentration of both ions of the same redox couple. The applicability of the method for practical purposes has been checked determining redox potential and concentration profiles for iron and reduced sulphur species in the pore-waters of sediments of the lagoon of Venice, Italy. (C) 2002 Elsevier Science B.V. All rights reserved.