Potentiometric study of acidic corrosive environments containing oxidative cations

The article presents data on the redox properties of solutions of mineral acids (H₂SO₄, HCl, H₃PO₄) and their mixtures (H₂SO₄+H₃PO₄, HCl+H₃PO₄) containing Fe(III) and Fe(II) cations with a total concentration of 0.1 M. For these systems, the values of the electrode potentials of the Fe(III)/Fe(II) redox couple were measured in the temperature range of 20−95°C using potentiometry on a platinum electrode. The redox potentials of systems in the solutions in question are poorly described by the Nernst equation. The observed deviations result from the non-equivalent complexation of potential-determining species, namely Fe(III) and Fe(II) cations, with anions present in a solution due to dissociation of the acids. The active concentrations of Fe(III) and Fe(II) cations in these systems change non-equally, which affects the redox potential of a system. The deviation of a system’s redox properties from the Nernst equation is the stronger the higher the complexing ability of the ligands formed from the acid. The greatest deviation of the experimentally determined values of the electrode potential of the Fe(III)/Fe(II) redox couple from those calculated using the Nernst equation based on the reference value of the standard electrode potential of this redox couple (E°Fe(III)/Fe(II) = 0.771 V (25°С)) is observed in environments containing H₃PO₄.

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