The hydrolysis of Al(III) in NaCl solutions-a model for Fe(III)

There is a lot of interest in the behavior of Fe(III) in natural waters because of its importance in the productivity of phytoplankton. The hydrolysis of Fe(III) in natural waters limits the solubility of Fe(III) in aqueous solutions. At the present time, reliable hydrolysis constants for Fe(III) are limited to low temperatures (5-50 °C) in NaCl to 5 m. The hydrolysis constants of Fe(III) and Al(III) are linearly related over a wide range of temperatures (0-50 °C) and ionic strengths (0-5 m NaCl). The near linear correlations allow one to make reasonable estimates for the values of Fe(III) from 0 to 300 °C in dilute solutions and from 0 to 100 °C to 5 m in NaCl solutions. In this paper, the stoichiometric (β _i) and thermodynamic (κ _i hydrolysis constants for Al(III) in NaCl have been fit to equations of the form logβ _i - logK _i=a ₀I ^0.5 + a ₁I ^0.5/T + a ₃I + a ₄ I / T + a ₅I ² This equation has been used to estimate the hydrolysis constant for Fe(III) in dilute solutions to 300 °C and in NaCl solutions to 5 m and 100 °C. The cause of the correlation in water is due to the differences in the free energies of ΔG ⁰(A ¹³⁺) - ΔG ⁰ (Al(OH) _j ^(3-j)) being almost equal to the values of ΔG 0 (Fe ³⁺) - ΔG ⁰ (Fe(OH) _j ^{(3- j))}. An examination of the activity coefficients of Fe ³⁺ and Al ³⁺ and the complexes show that the correlation at higher ionic strengths is due to the fact that the activity coefficients ratios are similar γ(Fe ³⁺) / γ(Fe(OH) _j ^(3-j))≅ γ(Al ³⁺) / γ(Al(OH) _j ^(3-j)) in NaCl solutions. These linear correlations appear to also hold for other trivalent metals (Cr ³⁺, As ³⁺). The results of this study should be useful in examining the speciation of Fe(III) in hydrothermal brines.