Feasibility Study for Determination of Trace Iron in Red Sandstone via O-Phenanthroline Spectrophotometry

Fe3+ and Fe2+ are widely present in red sandstone. However, systematic studies on the establishment of a quantitative relationship between the Fe3+/Fe2+ ratio and weathering degree of rock are scarce. In this study, on the basis of the coexistence characteristics of Fe2+ and Fe3+, as well as the ability of Fe2+ to form a stable orange–red complex with o-phenanthroline, an optimized o-phenanthroline spectrophotometric method for the multitarget determination of total iron, Fe2+, and Fe3+ was proposed and used to measure trace iron in the vertical profile of red sandstone. The effectiveness and reliability of the proposed method were validated via X-ray fluorescence spectroscopy (XRFS) and potassium dichromate titration. The results demonstrate that o-phenanthroline spectrophotometry offers advantages such as a low detection limit, high precision, and simple operation for the determination of trace iron in red sandstone. The vertical distribution pattern of the Fe2+/Fe3+ ratio is generally consistent with the macroscopic weathering intervals indicated by traditional chemical weathering indices. These findings suggest that the Fe2+/Fe3+ ratio has the potential to characterize vertical weathering zones and can serve as a simple auxiliary indicator for the rapid preliminary identification and classification of the relative weathering zones of red sandstone.