An Improved Procedure for the Determination of Ferrous Iron Mass Fraction in Silicate Rocks Using a Schlenk Line‐Based Digestion Apparatus to Exclude Oxygen

Abstract

Precise and accurate determination of ferrous iron mass fraction in silicate rocks and geological reference materials is still a significant challenge due to the labile nature of the analyte. Here, we report a modified and improved procedure for the determination of ferrous iron mass fraction, capable of yielding moderately accurate and highly precise results in a relatively shorter time than before. This modified technique requires a more convenient operational procedure than the classical method. A sample digestion apparatus was designed that included a Schlenk line to expel air in order to prevent oxidation. The advantages of this procedure over the classical procedure are shorter process flow, higher efficiency and absence of an additional redox indicator. The composition of the acid matrix used to dissolve the samples as well as the incubation temperature and time was investigated. Comparative data for twenty‐one reference materials are reported, with five of them reported for the first time by the Schlenk line procedure. A series of experiments were carried out to identify and minimise the main source of error. In addition, the possibility of eliminating the reducing substances before digestion, by pretreatment of the sample by cold 10% methanolic bromine, was also studied.

Online Flow‐Based Spectrophotometric Determination of Ferrous Iron Mass Fraction and Total Iron Mass Fraction in Twenty‐Seven Rock Reference Materials Using a Schlenk line Technique

“…The vessel was subjected to a vacuum to remove gases and then refilled with inert gas by rotating the stopcock, depending on the orientation. This cycle was repeated three times to ensure an inert atmosphere [for more detailed information on the Schlenkline, please see Xue (2017)]. Next, near-boiling H 2 SO 4 (9 mol l -1 , 0.6 ml) and HF (0.4 ml) were pipetted sequentially into the digestion vessel through the feed inlet, while Ar gas flowed at 1.0 l min -1 .…”

“…Therefore, the certified CRMs, JB-2 (basalt) and JG-2 (granite), were chosen to study the effects of variables on the sample decomposition of [Fe(II)] and [Fe(T)]. Based on our previous study, the optimum digestion temperature was 170°C because higher temperatures caused the PTFE to soften (Xue et al 2017). Therefore, the digestion temperature was not examined further.…”

“…In previous studies, 1:1 H 2 SO 4 (9 mol l -1 ) was favoured for dissolving the rocks (Terashima 1979, Yokoyama andNakamura 2002). According to Xue et al (2017), a concentration of 9 mol l -1 H 2 SO 4 is suitable for the dissolution of [Fe(II)] with low risk of oxidation, and therefore, this concentration was adopted in all subsequent studies. In Husler et al (2011), 5-14 mg samples were used.…”

“…The samples need to be dissolved, which significantly increases the difficulty of conducting the analysis. In this work, a custom-made sample digestion apparatus and heating block were designed, and a Schlenk line was introduced to expel air to prevent oxidation during sample digestion (as described by Xue et al (2017)). Then, the apparatus was connected to the continuous-flow injection analyser to implement the geological samples' online analysis.…”

Whole-rock and mineral chemical data from a profile of the ~900 Ma Niutishan Fe-Ti-rich sill in XuZhou, North China

“…Sample powders, ~0.5 g for each, were fused into a glass bead with ~5.0 g of lithium tetraborate. Major elements were measured using a PANalytical AXIOS Minerals instrument, and ferrous iron was measured by titration [2] ( Table S1 ). The uncertainties for major elements were<1.0 wt%.…”