We have developed a highly precise method for the determination of ferrous iron (Fe2+) in silicate rocks. Our new method is based on Wilson’s procedure (1955) in which surplus V5+ is used to oxidize Fe2+ into Fe3+ while equivalently reducing V5+ into V4+. Because V4+ is more resistant to atmospheric oxidation than Fe2+, Fe2+ in the sample can be determined by measuring unreacted V5+ by adding excess Fe2+ after sample decomposition and then titrating the unreacted Fe2+ with Cr6+. With our method, which involves conditioning the sample solution with 5 M H2SO4 in a relatively small beaker (7 mL), the oxidation of Fe2+ or V4+ that leads to erroneous results can be completely avoided, even in 100-h sample decompositions at 100°C. We have measured the concentration of FeO in 15 standard silicate rock powders provided by the Geological Survey of Japan (GSJ). Analytical reproducibility was better than 0.5% (1σ) for all but those samples that had small amounts of Fe2+ ( < 1.5 wt.% of FeO). Fourteen of these samples gave FeO contents significantly higher than the GSJ reference values. This likely indicates that the GSJ reference values, obtained by compiling previously published data, contain a large number of poor-quality data obtained by methods with lower recovery of Fe2+ caused by oxidation or insufficient sample decomposition during analyses. To achieve accurate determinations of Fe2+ in our method, several factors besides the oxidation must be considered, including: (1) long-term variations in the concentration of Fe2+ solution must be corrected; (2) excess use of the indicator must be avoided; and (3) the formation of inert FeF+ complex must be avoided during titration when using boric acid as a masking agent.
@article{20181025122944-742474,
author = "Tetsuya Yokoyama and Eizo Nakamura",
title = "Precise determination of ferrous iron in silicate rocks",
journal = "Geochimica et Cosmochimica Acta",
year = "2002",
number = "6",
volume = "66",
pages = "1085-1093",
doi = "10.1016/S0016-7037(01)00809-2",
}
caption | ID | rowname | stone | ||
---|---|---|---|---|---|
Table 2. FeO (wt.%) of GSJ standard rocks pub | 20191009135142-150123 | FeO (Yokoyama et al., 2002) | 7 | ||