Dissociation of Ferroin in acidic solutions and Nafion® membranes Original Research Article

The rates of dissociation of Ferroin, tris-(1,10-phenanthroline)iron(II) in acidic hydrochloric solutions and within Nafion® membranes (H+ and Na+ forms) were determined at 25°C and were found to be first order, acid independent reactions. The rate determining step for both solution and membrane media involved the detachment of the first 1,10-phenanthroline ligand with subsequent rapid loss of the remaining two ligands. Values of the averaged rate constants of the rate determining reaction in solutions and in the Nafion® membranes at constant ionic strength were 2.68×10−3 and 3.82×10−3 min−1, respectively, the difference being due to the steric influence of the membrane. These results will be utilised in overcoming stability problems encountered in a recently developed optical redox sensor based on Ferroin immobilised within a Nafion® membrane.