Quantitative determination of Mn ³⁺ /Mn ⁴⁺ in Mn oxide minerals through a titration method

Many manganese oxide (MnO x ) minerals incorporate both Mn 3+ and Mn 4+ in their structures. The ratio of these two Mn valences influences the absorption capacity of metal ions, the mechanisms of redox reaction, and topological transformations. Knowledge of the average oxidation state (AOS) of Mn is also significant in the case of biogenic or purely inorganic formation of these minerals. Enumeration of each oxidation state is still not a common part of their characterization, even though the AOS provides crucial information. In this work, we describe an accessible titration method based on the standard bismuthic method and quantify Mn 3+ /Mn 4+ in six secondary MnO x natural samples. The titration method was confirmed with XANES spectroscopy. This method does not require sophisticated laboratory equipment and generates sufficient accuracy and precision (relative error <1 %).