Speciation in the Fe(III)-Cl(I)-H₂O System at 298.15 K, 313.15 K, and 333.15 K (25 °C, 40 °C, and 60 °C)

This article presents the results on speciation of ferric iron generated by the dissolution of chemical reagent hydromolysite (ferric chloride hexahydrate, FeCl₃:6H₂O) in water at 298.15 K, 313.15 K, and 333.15 K (25 °C, 40 °C, and 60 °C). Experiments were performed with a thermoregulated system up to the equilibrium point, as manifested by solution pH. Solution samples were analyzed in terms of concentration, pH, and electrical conductivity. Measurements of density and refractive index were obtained at different temperatures and iron concentrations. A decrease of pH was observed with the increase in the amount of dissolved iron, indicating that ferric chloride is a strong electrolyte that reacts readily with water. Experimental results were modeled using the hydrogeochemical code PHREEQC in order to obtain solution speciation. Cations and neutral and anion complexes were simultaneously present in the system at the studied conditions according to model simulations, where dominant species included Cl⁻, FeCl²⁺, FeCl₂ ⁺, FeOHCl ₂ ⁰ , and H⁺. A decrease in the concentration of Cl⁻ and Fe³⁺ ions took place with increasing temperature due to the association of Fe-Cl species. Standard equilibrium constants for the formation of FeOHCl ₂ ⁰ obtained in this study were log \( K_{f}^{0} \) = −0.8 ± 0.01 at 298.15 K (25 °C), −0.94 ± 0.02 at 313.15 K (40 °C), and −1.03 ± 0.01 at 333.15 K (60 °C).