Iron(III) Hydrolysis and Solubility at 25 °C

UV−vis spectrophotometric measurements, potentiometric titrations, and solubility measurements were performed to evaluate the hydrolysis constants for aqueous Fe(III) and the solubility of 2-line ferrihydrite over a wide concentration range (0−3 M NaClO4 and p[H+] 1.54−11.23). From these measurements, Fe3+ was found to hydrolyze to form FeOH2+, Fe2(OH)24+ , Fe(OH)2+ , Fe(OH)30, and Fe(OH)4- . The hydrolysis and solubility constants of these species were determined together with their dependence on ionic strength. The iron(III) hydrolysis constants at infinity dilution were (logβ1,1 to logβ1,4 and logβ2,2) −2.19 ± 0.02, −5.76 ± 0.06, −14.30 ± 0.32, −21.71 ± 0.24, and −2.92 ± 0.02, respectively. The solubility product for 2-line ferrihydrite was (logKs,0) +3.50 ± 0.20. The results have been compared with literature values.