Some Thermodynamic and Kinetic Properties of the H2SO4–H3PO4–H2O–Fe(III) System

Abstract To characterize the oxidizing ability of the H2SO4–H3PO4–H2O–Fe(III) system, we measured the electrode potentials of the Fe(III)/Fe(II) redox couple and the half-wave potentials of the Fe3+ + e– = Fe2+ and Fe2+ – e– = Fe3+ transitions (reactions) on the cyclic voltammogram of a platinum electrode in acid solutions containing Fe(III) salts. It is shown that the values of these experimentally obtained parameters are close. The decrease in the oxidizing ability of H2SO4 and H3PO4 mixtures containing Fe(III) with an increase in the molar fraction of H3PO4 in them occurs due to the formation of Fe(III) complexes with phosphate anions, which are inferior in oxidizing ability to their hydrate and sulfate complexes. The temperature coefficients of the electrode potential (dE/dt) of the Fe(III)/Fe(II) redox couple in the H2SO4–H2O, H2SO4–H3PO4–H2O, and H3PO4–H2O systems were experimentally determined. Based on the Randles–Sevcik equation, the diffusion coefficients of Fe(III) in the studied solutions were calculated. The temperature dependence of the diffusion coefficients of Fe(III) cations is satisfactorily described by the Arrhenius equation. The parameters of this equation are calculated.