DETERMINATION OF AQUEOUS FE-III/II ELECTRON SELF-EXCHANGE RATES USING ENRICHED STABLE ISOTOPE LABELS, ION CHROMATOGRAPHY, AND INDUCTIVELY COUPLED PLASMA MASS SPECTROMETRY

摘要

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 Fe-57-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 HC1O(4) eluent. The resulting time series of Fe-56 and Fe-57 chromatograms display systematic changes in isotopic abundances from which the self-exchange rate constant, kll, is obtained. Two different schemes are used for mixing and sampling the reaction mixture; using a peristaltically pumped now reactor, reaction half-lives on the order of 30 s can be monitored. A series of kll results are obtained under a variety of temperature (2.0, 21.6, and 25.0 degrees 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.