Advances in iron(III) hydrolysis studies. Effect of the metal concentration, ionic medium and ionic strength

The hydrolytic behaviour of iron(III) was studied in absence and in presence of different salt solutions, in the ionic strength range: 0 < I/moldm−3 ≤ 1.0 in NaNO3(aq) and NaClO4(aq), and 0 < I/moldm−3 ≤ 3.0 in NaCl(aq), at T = 298.15 ± 0.15 K, by means of potentiometry and UV–Vis spectrophotometry (only in NaClO4). The two techniques allowed to perform the study in a wide range of iron(III) concentration: 2.5 ≤ cFe3+/mmoldm−3 ≤ 195.6 for the first, while 0.04 ≤ cFe3+/mmoldm−3 ≤ 0.25, for the second one. The results obtained by potentiometric measurements in NaNO3(aq) and NaCl(aq), allowed to determine the speciation model: Fe(OH)2+, Fe(OH)2+, Fe2(OH)24+, Fe3(OH)45+ and Fe12(OH)342+. In the case of the spectrophotometric titrations in NaClO4(aq), the best results were obtained considering the species: Fe(OH)2+, Fe(OH)2+ and Fe(OH)3(aq); for each one, the molar absorptivity values were calculated at the various experimental conditions investigated. The dependence on the ionic strength of the hydrolytic constants was modelled by means of Debye-Hückel type equation and of the Specific ion Interaction Theory. The differences of the hydrolytic constants in NaCl(aq) and NaNO3(aq) were interpreted in terms of different interactions of the Fe3+ with the anions of the supporting electrolyte. This interaction can be neglected when a not interacting medium as NaNO3(aq) is used. This consideration allowed to extrapolate the formation constants at infinite dilution of the weak FeCl2+ and FeCl2+ complexes; by using experimental data, it was also possible to calculate the equilibrium constant of the mixed FeOHCl+ species.