A Kinetic Approach to the Nature of Ferrous Ions in Aqueous Solution

Abstract

ION association involving ferrous ions is difficult to detect by the usual techniques: direct spectrophotometry due to the low extinction coefficients of Fe(II) species and the possible presence of trace Fe(III) species with high extinction coefficients; conductivity due to the high concentrations of anions needed; electrometric measurements due to the easier formation of complex species involving Fe(III) ions. In these circumstances, kinetics can be useful in providing information on complex formation, if a suitable reaction can be found. Such a reaction to study the nature of Fe(II) ions is that of Fe(II) with hydrogen peroxide: with [Fe(II)]/[H₂O₂] > 2, the reaction is first order in [Fe(II)] and first order in [H₂O₂] (refs. 1 and 2): We have found that the reaction is independent of ionic strength, independent of hydrogen ion concentration below pH 3 using perchloric acid and that the rate of the slow step (1) is dependent on the charge on the Fe(II) species. This allows the determination of the concentrations of free and of associated Fe (II) from the observed rate: and ionic strength effects must be due to the existence of equilibria between various Fe(II) species.