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Metallic iron (Fe⁰) materials have been industrially used for water treatment since the 1850s. There are still many fundamental challenges in affordably and reliably characterizing the Fe⁰ intrinsic reactivity. From the available methods, the one using Fe⁰ dissolution in ethylenediaminetetraacetic acid (EDTA—2 mM) was demonstrated the most applicable as it uses only four affordable chemicals: Ascorbic acid, an ascorbate salt, EDTA and 1,10-Phenanthroline (Phen). A careful look at these chemicals reveals that EDTA and Phen are complexing agents for dissolved iron species. Fe³-EDTA is very stable and difficult to destabilize; ascorbic acid is one of the few appropriate reducing agents, therefore. On the other hand, the Fe²-Phen complex is so stable that oxidation by dissolved O₂ is not possible. This article positively tests Fe⁰ (0.1 g) dissolution in 2 mM Phen (50 mL) as a characterization tool for the intrinsic reactivity, using 9 commercial steel wool (Fe⁰ SW) specimens as probe materials. The results are compared with those obtained by the EDTA method. The apparent iron dissolution rate in EDTA (k_EDTA) and in Phen (k_Phen) were such that 0.53 ≤ k_EDTA (μg h⁻¹) ≤ 4.81 and 0.07 ≤ k_Phen (μg h⁻¹) ≤ 1.30. Higher k_EDTA values, relative to k_Phen, are a reflection of disturbing Fe³ species originating from Fe² oxidation by dissolved O₂ and dissolution of iron corrosion products. It appears that the Phen method considers only the forward dissolution of Fe⁰. The Phen method is reliable and represents the most affordable approach for characterizing the suitability of Fe⁰ for water treatment.