The dependence of FeIII hydrolysis on ionic strength in NaCl solutions

The influence of ionic strength on FeOH2+ formation (0.1 m≤I≤6.0 m) in NaCl solutions at 25 °C was observed potentiometrically, and can be summarized aslogB1*=−2.34(±0.01)−2.044I1/2/(1+1.70(±0.08)I1/2)−0.228(±0.005)Iwhere B1*=[FeOH2+][H+][FeIII]−1, brackets denote species concentrations, and [FeIII] is the total FeIII concentration ([FeIII]T) exclusive of hydrolyzed forms of FeIII ([FeIII]=[Fe3+]+[FeCl2+]+[FeCl2+]). The magnitude of B1* is well described in terms of FeIII hydrolysis constants (β1*) determined in NaClO4 solutions (β1*=[FeOH2+][H+][Fe3+]−1), and ferric chloride formation constants determined in moderately acidic solutions (Clβ1=[FeCl2+][Fe3+]−1[Cl−]−1, Clβ2=[FeCl2+][Fe3+]−1[Cl−]−2):logβ1*=−2.179(±0.012)−2.044I1/2/(1+2.42(±0.11)I1/2)+0.020(±0.004)IlogClβ1=1.26(±0.01)−3.066I1/2/(1+2.10(±0.06)I1/2)+0.130(±0.003)IlogClβ2=2.53(±0.16)−5.11I1/2/(1+0.86(±0.22)I1/2)+0.27(±0.12)IFerric chloride formation constants are strongly medium dependent. At high ionic strengths, FeCl2+ formation constants are much larger in HClO4 solutions than in NaClO4 solutions. In strong acids (HClO4+HCl) log Clβ1 is given aslogClβ1=1.26(±0.04)−3.066I1/2/(1+1.78(±0.11)I1/2)+0.244(±0.006)IChloride complexation exerts a strong influence on the magnitude of B1*. In 0.7 m NaCl, B1* is smaller than β1* by a factor of three, and in 6 m NaCl, B1* is smaller than β1* by a factor of eighty. Chloride complexation also exerts a strong influence on the enthalpy of FeIII hydrolysis. While observations of the influence of temperature on β1* in 0.72 m NaClO4 indicate that the enthalpy for the FeIII hydrolysis reaction is ΔH=42.7±0.8 kJ mol−1 [Byrne, R.H., Luo, Y.R., Young, R.W., 2000. Iron hydrolysis and solubility revisited: observations and comments on iron hydrolysis characterizations. Mar. Chem. 70, 23–35], observations of B1* in 0.7 m NaCl indicate that ΔH=26.0±1.2 kJ mol−1. This result is in good agreement with results obtained via solubility analysis [Liu, X., Millero, F.J., 1999. The solubility of iron hydroxide in sodium chloride solutions. Geochim. Cosmochim. Acta 63, 3487–3497]. Potentiometric observations of FeIII solubility equilibria in 0.7 m NaCl at 25 °C can be summarized aslog[Fe3+]=4.62(±0.06)−2.73(±0.01)pHThe slope (n=−2.73) obtained for freshly precipitated hydrous ferric oxides in 0.7 m NaCl is identical to that observed in previous work [Biedermann, G., Chow, J.T., 1966. Studies on the hydrolysis of metal ions. Acta Chem. Scand. 20 (5), 1376–1388] using aged precipitates in 0.5 m NaCl. This slope is somewhat smaller than that observed in 0.7 m NaClO4 at 25 °C [n=−2.86±0.009; Byrne, R.H., Luo, Y.R., 2000. Direct observations of nonintegral hydrous ferric oxide solubility products: Kso*=[Fe3+][H+]−2.86. Geochim. Cosmochim. Acta. 64, 1873–1877.].