Cyclic voltammetric study of the HCl–H3PO4–H2O–Fe(III) system

The electrochemical behavior of Fe(III) cations on a platinum electrode in the HCl-H3PO4-H2O, HCl-H2O, H3PO4-H2O systems (25-95°C) has been studied by cyclic voltammetry.The oxidizing ability of the 2.0 M HCl+H3PO4 system containing 0.10 М Fe(III) 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 chloride complexes.An increase in temperature causes an increase in the oxidizing ability of the HCl-H3PO4-H2O-Fe(III) system.Addition of H3PO4 to HCl solutions converts Fe(III) cations from hydrate and chloride complexes to phosphate complexes, which leads to a decrease in their diffusion rate in aqueous acid solutions.The decrease in the diffusion coefficient of Fe(III) cations in 1.0 M HCl+1.0M H3PO4 is about 70% of the initial value.A probable reason for more efficient protection of low-carbon steel by triazole-based composite inhibitors in H3PO4 or its mixtures with HCl containing Fe(III), in comparison with similar solutions of HCl alone, is that phosphate complexes of Fe(III) are formed in these media.They have a lower oxidizing ability and diffusion rate in comparison with aqueous and chloride complexes of Fe(III).