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

Abstract

Abstract 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 FeOH 2+ hydrolysis constant characterizations to 6 M ionic strength. These results obtained in NaClO 4 are in excellent agreement with previous spectrophotometric characterizations over the range of conditions of those previous works. The enthalpy for the first Fe 3+ hydrolysis step (Δ H 1 *) is 10.2 kcal mol −1 and the molal scale hydrolysis constant appropriate to the first hydrolysis step ( β 1 *=[FeOH 2+ ][H + ][Fe 3+ ] −1 ) is log β 1 *=−2.71±0.03 at 25°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 ( β 2 *=[Fe(OH) 2 + ][H + ] 2 [Fe 3+ ] −1 ) for ionic strengths between 0.7 and 3.0 M are on the order of log β 2 *≤−7. This constant is, like β 1 *, expected to be strongly temperature-dependent (Δ H 2 *≅16.9 kcal mol −1 ). Previous estimates for β 3 * derived from solubility analyses are operationally defined and model-dependent. At 25°C and the ionic strength of seawater β 3 * is estimated as log β 3 *≤−13.6. Enthalpy estimates appropriate to β 3 * (Δ H 3 *≅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 *=[Fe 3+ ][H + ] − n , should be written with n