A potentiometric study of an H2SO4–H3PO4–H2O system containing Fe(III) and Fe(II) cations

• Ya.G. Avdeev, T.E. Andreeva and Yu.I. Kuznetsov

A.N. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow, 119071 Russian Federation

Abstract: The potentiometric method was used to study an H2SO4–H3PO4–H2O system (20–95°C) containing Fe(III) and Fe(II) cations. The oxidizing ability of the 2 M H2SO4 + H3PO4 system containing Fe(III) and Fe(II) (1:1) decreases as the molar fraction of H3PO4 increases. The observed effect results from the formation of Fe(III) complexes with phosphate anions that are weaker oxidants than the corresponding hydrate and sulfate complexes. Variation in the total content of the equimolar Fe(III) + Fe(II) mixture (C = 0.01–0.10 M) in 1 M H2SO4 + 1 M H3PO4 nearly does not affect its oxidizing ability. Conversely, an increase in the relative content of Fe(III) in the system considerably increases its oxidative potential. At a fixed concentration of the Fe(III) + Fe(II) mixture in the system (0.04 М), addition of an equimolar mixture of H2SO4 and H3PO4 (up to 6 M) somewhat decreases the oxidizing properties of the system. A probable reason of the efficient protection of low-carbon steel by triazole-based composite inhibitors in H3PO4 or its mixtures with H2SO4 containing Fe(III), in comparison with similar solutions of H2SO4 alone, is that phosphate complexes of Fe(III) are formed in these media. They have lower oxidizing ability in comparison with aqueous and sulfate complexes of Fe(III).

Keywords: corrosion inhibitors, potentiometry, sulfuric acid, phosphoric acid, Fe(III)/Fe(II) redox couple

Int. J. Corros. Scale Inhib., 2018, 7, no. 3, 366-375 PDF (327 K)
doi: 10.17675/2305-6894-2018-7-3-7

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