The Assay for Ferric and Ferrous Iron

In order to study iron reactivity qualitatively and quantitatively it is essential to quantify the ferrous and ferric iron fractions of the present minerals or mineral groups. With respect to the determination of iron speciation the principal problem is the rapid oxidation of ferrous iron. Atmospheric oxygen diffuses into pore water where it oxidizes dissolved ferrous iron immediately and starts oxidizing FeS and FeS. Reduced smectites may also become oxidized under air atmosphere within hours. Therefore, if dissolved iron and iron speciation of solid phase are to be determined samples need to be conserved under inert gas atmosphere. No extra care is needed for the determination of total iron of solid phase. 

Above a pH of 3 and in the absence of chelators dissolved iron is only present as ferrous iron under natural conditions. Therefore, the colored complex that results from the reaction between ferrous iron and a reagent can be analysed colorimetrically and correlates with the concentration of total dissolved iron. Most conveniently, one can mix a drop of Ferrozine solution (Stookey 1970), one drop of H SO (diluted 1 4) and 1 ml of pore water in the glove box, wait until complex formation is completed (20-30 minutes) and quantify the iron concentration by the intensity of the color at a wavelength of 562 nm. To avoid matrix effects standards should be prepared with artificial seawater. 

The assay of solid phase ferrous and ferric iron has a long tradition due to the early interest in soil chemistry. Publications on extraction / leaching conditions and results from varying soils and sediments are extensive and thus, within the scope of a textbook, only important principals and a description of the (subjectively) most important extractions can be given. Since a 

Schwertmann (1964) 0.2 M NH4+-oxalate / 0.2 M oxalic acid pH 2.5, 2 h in darkness Ferdelman (1988) 10 g Na-citrate + 10 g Na-bicarbonate mixed in 200 ml distilled and deionized water, deaerated, before 4 g ascorbic acid are added pH 7.5, 24 h 

Chou and Zhou 1983, Canfield 1988, Kostka and Luther 1994, Ruttenberg 1992, I l Mehra and Jackson 1960, 

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