A New 1,10-Phenanthroline Method for Oxalate-extractable Iron Measurement

Iron separation from different solid phases is essential for evaluating the environmental function of iron in sediments. Oxalate is a commonly used extractant that effectively extract iron(hydro)oxides in sediments by complexation. However, when using spectrophotometry method, excess oxalate will interfere the complexation of iron with 1,10-phenanthroline resulting in the failure of iron measurement. In this study, we discovered an effective method for spectrophotometric analysis of iron samples with oxalate by adjusting the pH to 7-9, which changes the structure of the Fe-oxalate complexes and ensured the complexation of iron with 1,10-phenanthroline. Further exploration indicates that photolysis and heating also decompose Fe-oxalate complexes, but the performance is not as good as pH adjustment. The standard solution exhibits a strong linear relationship between absorbance (Abs) and concentration (Con): Abs = 0.1934 × Con + 0.1360, with a R² of 0.9997, accuracy of 97.1%, and precision of 98.6%, which demonstrate the reliability of this method. Overall, the pretreatment is simple, and the influence of the organic solvent (oxalate) is diminished after pH adjustment. This method is expected to contribute to community by providing a new reliable, effective, less pre-treatment, costive, and sensitive testing approach and hopefully assist in the investigation on environmental function of iron minerals.