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

To characterize the oxidizing ability of the HCl—H 3 PO 4 –H 2 O–Fe(III) system, the electrode potentials of the Fe(III)/Fe(II) redox couple and the half-wave potentials of transitions Fe 3+ + e – = Fe 2+ and Fe 2+ – e – = Fe 3+ on a cyclic voltammogram of a platinum electrode in acid solutions containing Fe(III) salts have been measured. It is shown that the values of these experimentally obtained parameters are close. A decrease in the oxidizing ability of the mixtures of HCl and H 3 PO 4 containing Fe(III) with an increase in the molar fraction of H 3 PO 4 occurs due to the formation of complexes of Fe(III) with phosphate anions, which are inferior in oxidizing ability to their hydrate and chloride complexes. Temperature coefficients of the electrode potential (d E /d t ) of the Fe(III)/Fe(II) redox couple in the HCl—H 2 O, HCl—H 3 PO 4 –H 2 O, and H 3 PO 4 –H 2 O systems have been experimentally determined. Based on the Randles–Shevchik equation, the diffusion coefficients of Fe(III) in the solutions under study 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.