Optimizing the Pratt-type titrimetric method to determine FeO in geochemical reference materials

Abstract

Determination of ferrous iron (FeO) is important across a wide breadth of disciplines in geochemistry. The certified values of FeO in geochemical reference materials have been determined mainly by wet chemistry. During this procedure, special attention is needed to prevent oxidation of Fe²⁺ to Fe³⁺, which may lead to underestimations of FeO mass fractions. Here, we revisited a traditional titrimetric method to provide a better method of determining the FeO mass fractions in geochemical reference materials. We investigated the effect of duration of decomposition, sand bath temperature, and sample weight. Among these, we found that the effect of sample weight on the FeO values is most significant. The FeO values increase with increasing sample weight, indicating that vapor and foam effectively prevent oxidation. This is supported by yield tests and experiments involving the addition of SiO₂ powder to the samples. Thus, we suggest the use of greater amounts of sample to minimize oxidation. We further compare our results with the published recommended values for reference materials, demonstrating that the latter need be considered carefully. The approach described here is not only applicable to geochemical reference materials but also useful to unknown geological samples.