Iron hydrolysis and solubility revisited: observations and comments on iron hydrolysis characterizations

This work describes and evaluates previous hydrolysis characterizations relevant to the behavior of iron in natural solutions, with special emphasis given to the physical chemistry of iron in seawater. New potentiometric data are used in this work to extend FeOH2+ hydrolysis constant characterizations to 6 M ionic strength. These results obtained in NaClO4 are in excellent agreement with previous spectrophotometric characterizations over the range of conditions of those previous works. The enthalpy for the first Fe3+ hydrolysis step (Delta H-1*) is 10.2 kcal mol(-1) and the molal scale hydrolysis constant appropriate to the first hydrolysis step (beta(1)* = [FeOH2+][H+][Fe3+](-1)) is log beta(1)* = -2.71 +/- 0.03 at 25 degrees C and the ionic strength of seawater. Recalculations of previous data, as well as new experimental analyses in this work indicate that Fe(OH)(2)(+) hydrolysis constants (beta(2)* = [Fe(OH)(2)(+)][H+](2)[Fe3+](-1)) for ionic strengths between 0.7 and 3.0 M are on the order of log beta(2)* less than or equal to -7. This constant is, like beta(1)*, expected to be strongly temperature-dependent (Delta H-2* congruent to 16.9 kcal mol(-1)). Previous estimates for beta(3)* derived from solubility analyses are operationally defined and model-dependent. At 25 degrees C and the ionic strength of seawater beta(3)* is estimated as log beta(3)* less than or equal to - 13.6. Enthalpy estimates appropriate to beta(3)* (Delta H-3* congruent to 23.9 kcal mol(-1)) indicate that this upper bound decreases strongly with decreasing temperature. Our current understanding of the pH dependence of Fe-III solubility behavior is closely tied to assumptions about the form of Fe(OH)(3(s)) solubility products in aqueous solution. Evidence is presented which indicates that the Fe(OH)(3(s)) solubility product, K-so* = [Fe3+][H+](-n), should be written with n < 3. (C) 2000 Elsevier Science B.V. All rights reserved.