Determination of Aqueous Fe^III/III Electron Self-Exchange Rates Using Enriched Stable Isotope Labels, Ion Chromatography, and Inductively Coupled Plasma Mass Spectrometry

A strategy is described for rapid on-line measurement of electron self-exchange rates between aqueous Fe^III and Fe^II in aqueous solution using stable ⁵⁷Fe-labeled reactants, cation chromatography, and inductively coupled plasma mass spectrometry. The self-exchange is monitored by mixing the reactants and performing timewise separations of Fe^II and Fe^III ions. Separations are completed in 30-60 s using a weak phosphonic/carboxylic acid cation exchange guard column and 0.1-0.5 M aqueous HClO₄ eluent. The resulting time series of ₅₆Fe and ₅₇Fe chromatograms display systematic changes in isotopic abundances from which the self-exchange rate constant, k₁₁, is obtained. Two different schemes are used for mixing and sampling the reaction mixture; using a peristaltically pumped flow reactor, reaction half-lives on the order of 30 s can be monitored. A series of k₁₁ results are obtained under a variety of temperature (2.0, 21.6, and 25.0 °C) and ionic strength conditions (0.10, 0.50, and 0.55 M aqueous HClO₄) which are congruent with three previously published radiolabeling studies for this reaction.