Field sampling, speciation and determination of dissolved iron (II) and iron (III) in waters

A simple and rapid field sampling procedure was developed for the speciation of dissolved Fe(II) and Fe(III) in waters. The determination of iron species was possible by selective batch solid phase extraction of Fe(III) using chelating resin Chelex-100 in H+ form, sample acidity range of pH 1.5-2.5, elution with 0.03 mol L-1 NH4-EDTA, and detection of Fe(III) by flame or electrothermal atomic absorption spectrometry (ETAAS). The concentration of Fe(II) was determined in the solution above the resin by direct ETAAS or after adsorption on Chelex-100 in NH4+ form without the need for preoxidation of Fe(II) to Fe(III). Water samples were collected in situ and filtered by passing them through a syringe filter (0.45 mu m). The batch procedure was performed at the field and then, the tubes containing the resins with the loaded analytes were returned to the laboratory where the iron species were eluted and determined. Field sampling prevents changes in the oxidation state of iron. The effect of humic acid was also investigated. The results obtained indicated that the method was not affected by the presence of up to 0.01% humic acid. The limit of detection (3s) was 0.8 mu g L-1 Fe (ETAAS detection). The relative standard deviation (n=10) ranged from 2% at the 1 mg L-1 Fe up to 20% at the 1 mu g L-1 Fe(III) level. Recoveries of spiked Fe(II) and Fe(III) in river, lake, tap and groundwater samples ranged from 93 to 105%.