About the observance of the Nernst equation in acid corrosive media containing oxidative cations

• Ya.G. Avdeev¹, T.E. Andreeva¹, K.L. Anfilov² and Yu.I. Kuznetsov¹

¹ A.N. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Leninsky pr. 31, 119071 Moscow, Russian Federation
² Bauman Moscow State Technical University (Kaluga Branch), Bazhenov str. 2, 248000 Kaluga, Russian Federation

Abstract: Potentiometry on a platinum electrode in the temperature range of 20–95°C was used to study the redox properties of 2 М H₂SO₄, 2 М HCl, 2 М H₃PO₄, 1 М H₃PO₄ + 1 М H₂SO₄, and 1 М H₃PO₄ + 1 М HCl solutions containing Fe(III) and Fe(II) salts. 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. Among the media studied, the greatest deviation of the redox properties of the Fe(III)/Fe(II) redox couple from the theoretical values is observed in media containing H₃PO₄. The observed effects should be taken into account in the creation and operation of corrosion inhibitors for low-carbon steels in mineral acid solutions containing Fe(III) and Fe(II) salts, since their efficiency considerably depends on the oxidative capability of these environments.

Keywords: Nernst equation, potentiometry, hydrochloric acid, sulfuric acid, phosphoric acid, Fe(III)/Fe(II) redox couple, iron(III) chloride, iron(III) sulfate, iron(III) phosphate, corrosion inhibitors

Int. J. Corros. Scale Inhib., 2022, 11, no. 2, 752-761
doi: 10.17675/2305-6894-2022-11-2-20

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