Accurate determination of Fe(II) concentrations in the presence of a very high soluble Fe(III) background Youri Gendel, Ori Lahav * Faculty of Civil and Environmental Engineering, Technion – Israel Institute of Technology, Haifa, 32000, Israel article info Article history: Received 27 September 2007 Accepted 27 March 2008 Available online 16 May 2008 Editorial handling by R. Fuge abstract Analytical methods used for determining dissolved Fe(II) often yield inaccurate results in the presence of high Fe(III) concentrations. Accurate analysis of Fe(II) in solution when it is less than 1% of the total dissolved Fe concentration (Fe T ) is sometimes required in both geochemical and environmental studies. For example, such analysis is imperative for obtaining the ratio Fe(II)/Fe(III) in rocks, soils and sediments, for determining the kinetic constants of Fe(II) oxidation in chemical or biochemical systems operating at low pH, and is also important in environmental engineering projects, e.g. for proper control of the regeneration step (oxidation of Fe(II) into Fe(III)) applied in ferric-based gas desulphu- rization processes. In this work a method capable of yielding accurate Fe(II) concentrations at Fe(II) to Fe T ratios as low as 0.05% is presented. The method is based on a pretreatment procedure designed to separate Fe(II) species from Fe(III) species in solution without changing the original Fe(II) concentration. Once separated, a modified phenanthroline method is used to determine the Fe(II) concentration, in the virtual absence of Fe(III) spe- cies. The pretreatment procedure consists of pH elevation to pH 4.2–4.65 using NaHCO 3 under N 2 (g) environment, followed by filtration of the solid ferric oxides formed, and sub- sequent acidification of the Fe(II)-containing filtrate. Accuracy of Fe(II) analyses obtained for samples (Fe(II)/Fe T ratios between 2% and 0.05%) to which the described pretreatment was applied was >95%. Elevating pH to above 4.65 during pretreatment was shown to result in a higher error in Fe(II) determination, likely resulting from adsorption of Fe(II) species and their removal from solution with the ferric oxide precipitate. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction The commonly used analytical methods for determining dissolved Fe(II) concentration (e.g. the phenanthroline method) yield inaccurate results in the presence of a high Fe(III) background (Herrera et al., 1989). The need for a technique that yields an accurate Fe(II) analysis in cases where the Fe(II) concentration is less than 1% of Fe T arises particularly in crystallographic and geochemical studies (e.g. for obtaining the ratio between Fe(II) and Fe(III) in rocks, soils and sediments), and in studies concerned with determining the kinetic constants of Fe(II) oxidation in chemical or biochemical systems operating at low pH (Tufekci and Sarikaya, 1996; Nemati et al., 1998; Gomez and Cantero, 2003; Molchanov et al., 2007). The same anal- ysis is also needed in environmental engineering, for exam- ple in the context of the wide spread ferric-based gas desulphurization processes in which H 2 S (g) is oxidized to elemental S in an acidic (normally pH < 2) Fe(III) solution, and the generated Fe(II) is concurrently oxidized back to Fe(III) (typically using Acidithiobacillus ferrooxidans bacte- ria) to allow for steady state continuous operation (Asai et al., 1989; Ebrahimi et al., 2002). In both cases the rate of Fe(II) chemical or biological oxidation is highly depen- dent on the Fe(III) concentration, which is in many cases 2 or 3 orders of magnitude higher than the Fe(II) concentra- tion (Jones and Kelly, 1983; Nemati and Webb, 1998; 0883-2927/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2008.03.016 * Corresponding author. Tel.: +972 4 8292191; fax: +972 4 8228898. E-mail address: agori@techunix.technion.ac.il (O. Lahav). Applied Geochemistry 23 (2008) 2123–2129 Contents lists available at ScienceDirect Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem