The influence of ionic strength on FeOH 2+ formation (0.1 m ≤ I ≤ 6.0 m) in NaCl solutions at 25 °C was observed potentiometrically, and can be summarized as log B 1 * = − 2.34 ( ± 0.01 ) − 2.044 I 1 / 2 / ( 1 + 1.70 ( ± 0.08 ) I 1 / 2 ) − 0.228 ( ± 0.005 ) I where B 1* = [FeOH 2+][H +][Fe III] − 1 , brackets denote species concentrations, and [Fe III] is the total Fe III concentration ([Fe III] T) exclusive of hydrolyzed forms of Fe III ([Fe III] = [Fe 3+] + [FeCl 2+] + [FeCl 2 +]). The magnitude of B 1* is well described in terms of Fe III hydrolysis constants ( β 1*) determined in NaClO 4 solutions ( β 1* = [FeOH 2+][H +][Fe 3+] − 1 ), and ferric chloride formation constants determined in moderately acidic solutions ( Cl β 1 = [FeCl 2+][Fe 3+] − 1 [Cl −] − 1 , Cl β 2 = [FeCl 2 +][Fe 3+] − 1 [Cl −] − 2 ): log β 1 * = − 2.179 ( ± 0.012 ) − 2.044 I 1 / 2 / ( 1 + 2.42 ( ± 0.11 ) I 1 / 2 ) + 0.020 ( ± 0.004 ) I log Cl β 1 = 1.26 ( ± 0.01 ) − 3.066 I 1 / 2 / ( 1 + 2.10 ( ± 0.06 ) I 1 / 2 ) + 0.130 ( ± 0.003 ) I log Cl β 2 = 2.53 ( ± 0.16 ) − 5.11 I 1 / 2 / ( 1 + 0.86 ( ± 0.22 ) I 1 / 2 ) + 0.27 ( ± 0.12 ) I Ferric chloride formation constants are strongly medium dependent. At high ionic strengths, FeCl 2+ formation constants are much larger in HClO 4 solutions than in NaClO 4 solutions. In strong acids (HClO 4 + HCl) log Cl β 1 is given as log Cl β 1 = 1.26 ( ± 0.04 ) − 3.066 I 1 / 2 / ( 1 + 1.78 ( ± 0.11 ) I 1 / 2 ) + 0.244 ( ± 0.006 ) I Chloride complexation exerts a strong influence on the magnitude of B 1*. In 0.7 m NaCl, B 1* is smaller than β 1* by a factor of three, and in 6 m NaCl, B 1* is smaller than β 1* by a factor of eighty. Chloride complexation also exerts a strong influence on the enthalpy of Fe III hydrolysis. While observations of the influence of temperature on β 1* in 0.72 m NaClO 4 indicate that the enthalpy for the Fe III hydrolysis reaction is Δ H = 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 B 1* in 0.7 m NaCl indicate that Δ H = 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 Fe III solubility equilibria in 0.7 m NaCl at 25 °C can be summarized as log [ Fe 3 + ] = 4.62 ( ± 0.06 ) − 2.73 ( ± 0.01 ) pH The 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 NaClO 4 at 25 °C [ n = − 2.86 ± 0.009; Byrne, R.H., Luo, Y.R., 2000. Direct observations of nonintegral hydrous ferric oxide solubility products: K so*=[Fe 3+][H +] −2.86. Geochim. Cosmochim. Acta. 64, 1873–1877.].