The mixed and variable valence of iron in magnetite (Fe(III)tet[Fe(II),Fe(III)]octO42? ) give this 202 mineral unique properties that make it an important participant in redox reactions in 203 environmental systems, but the variability in its stoichiometry and other physical properties 204 complicates the determination of its effective redox potential. To address this challenge, a robust 205 method was developed to prepare working electrodes with mineral powders of diverse 206 characteristics and agarose-stabilized pore waters of controlled composition. This second-207 generation powder-disk electrode (PDEv2) methodology was used to characterize the 208 electrochemical properties of magnetite samples from a wide variety of sources (lab-synthesized, 209 commercial, and magnetically separated from environmental samples) using a sequence of 210 complementary potentiometric methods: chronopotentiometry (CP), linear polarization resistance 211 (LPR), electrochemical impedance spectroscopy (EIS), and then linear sweep voltammetry 212 (LSV).