A spectrophotometric study of iron(III) hydrolysis in aqueous solutions to 200 °C

The hydrolysis of iron(III) was studied in acid aqueous solutions between 25 and 200 °C at saturated water vapour pressure by uv-vis spectrophotometry using a high-temperature, flow-through gold-lined optical cell. The strong ligand-to-metal charge transitions of iron(III) hydroxo complexes at wavelengths below 400 nm were used to obtain molar absorptivities, ε, and equilibrium hydrolysis constants using principal component analysis of the spectra. The total iron(III) concentrations ranged from 6.184 × 10^{- 5} to 1.652 × 10^{- 4} mol kg^{- 1} and the perchloric acid concentration was between 9.31 × 10^{- 4} and 0.398 mol kg^{- 1}. Under these conditions two iron(III) species were identified, Fe³⁺ and FeOH²⁺, and the corresponding hydrolysis constant according to the reactionFe³⁺ + H₂O = FeOH²⁺ + H⁺increased from logβ₁ = - 2.18 ± 0.01 at 25 °C to 0.54 ± 0.15 at 200 °C. The hydrolysis of Fe³⁺ to form FeOH²⁺ is predominantly driven by positive entropy indicating electrostatic interaction between Fe³⁺ and OH^-.