Analysis of Ferric and Ferrous Ions in Soil Extracts by Ion Chromatography

Abstract

A method using ion chromatography (IC) for the analysis of ferrous (Fe ²⁺) and ferric (Fe ³⁺) ions in soil extracts has been developed. This method uses an ion exchange column with detection at 520 nm after post-column derivatization. Selectivity is achieved by using an anionic chelating agent, pyridine-2,6-dicarboxylic acid, to form anionic complexes with ferric and ferrous ions. Using this method, both ferric and ferrous ions can be analyzed directly and simultaneously. The soil extractions were carried out with both 0.5 M hydrochloric acid (HCl) and 0.36 M oxalate under anaerobic conditions. Ferric and ferrous ions were stable in either 0.5 M HCl solution or deionized water at pH ∼ 1.7 when stored outside of a glove box. Ferrous ion was readily oxidized to ferric ion in aqueous solutions at pH values above 4.0 and in 0.36 M oxalate solutions at all pH values.

In addition, we note that ferric ion in 0.5 M HCl was detectably reduced to ferrous ion when stored for 24 hours in a glove box containing a few percent of hydrogen gas. The study indicated that there was no interference in the HPLC analysis from the common cations Ca²⁺, Mg²⁺ and Al³⁺.