Predicting the redox behavior of magnetite in reducing soils and sediments is challenging because there is neither agreement among measured potentials nor consensus on which Fe(III) | Fe(II) equilibria are most relevant. Here, we measured open-circuit potentials of stoichiometric magnetite equilibrated over a range of solution conditions. Notably, electron transfer mediators were not necessary to reach equilibrium. For conditions where ferrous hydroxide precipitation was limited, Nernstian behavior was observed with an E_H vs pH slope of −179 ± 4 mV and an E_H vs Fe(II)_aq slope of −54 ± 4 mV. Our estimated E_H^o of 857 ± 8 mV closely matches a maghemite|aqueous Fe(II) E_H^o of 855 mV, suggesting that it plays a dominant role in poising the solution potential and that it’s theoretical Nernst equation of E_H[mV] = 855 – 177 pH – 59 log [Fe²⁺] may be useful in predicting magnetite redox behavior under these conditions. At higher pH values and without added Fe(II), a distinct shift in potentials was observed, indicating that the dominant Fe(III)|Fe(II) couple(s) poising the potential changed. Our findings, coupled with previous Mössbauer spectroscopy and kinetic data, provide compelling evidence that the maghemite/Fe(II)_aq couple accurately predicts the redox behavior of stoichiometric magnetite suspensions in the presence of aqueous Fe(II) between pH values of 6.5 and 8.5.

中文翻译：

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还原条件下磁铁矿悬浮液的氧化还原电位

预测磁铁矿在还原土壤和沉积物中的氧化还原行为具有挑战性，因为测量的电势之间既没有达成共识，也没有就哪个 Fe(III) | Fe(II) 平衡最相关。在这里，我们测量了在一系列溶液条件下平衡的化学计量磁铁矿的开路电位。值得注意的是，电子转移介体并不是达到平衡所必需的。对于氢氧化亚铁沉淀受限的条件，观察到能斯特行为，E _H与 pH 的斜率为 -179 ± 4 mV，E _H与 Fe(II) _aq的斜率为 -54 ± 4 mV。我们估计的E _H ^o为 857 ± 8 mV，与磁赤铁矿|含水 Fe(II) E非常匹配_H ^o为 855 mV，表明它在平衡溶液电势方面起着主导作用，并且E _H [mV] = 855 – 177 pH – 59 log [Fe ²⁺ ] 的理论能斯特方程可用于预测磁铁矿氧化还原这些条件下的行为。在较高的 pH 值和不添加 Fe(II) 的情况下，观察到明显的电位变化，表明平衡电位的主要 Fe(III)|Fe(II) 对发生了变化。我们的发现，结合之前的穆斯堡尔光谱和动力学数据，提供了令人信服的证据，证明磁赤铁矿/Fe(II)_水性偶联准确预测了在 pH 值介于 6.5 和 8.5 之间的水性 Fe(II) 存在下化学计量磁铁矿悬浮液的氧化还原行为.