Reaction entropies and acid-base behavior of transition-metal complexes in recast Nafion films

Abstract

The effect of Donnan potentials on the apparent formal potentials of polymer-bound metal ions has been investigated for recast Nafion films and for spin-cast Nafion films on graphite electrodes. For the probe ions Ru(NH₃)₆^(3+/2+) and Ru(bpy)₃^(3+/2+), the behavior generally correlates with Donnan potentials expected for ideally permselective domains, and plots of the apparent E⁰' vs ionic strength (µ) have slopes of 59 mV/(decade increase in µ) for 1:1 monovalent electrolytes. The observed reaction entropies, ΔS_(obsd), have been determined as a function of solution ionic strength. The measured ΔS_(obsd) values are found to consist of the intrinsic reaction entropy for the reaction and an entropy of transfer term. The latter term arises from the permselectivity properties of the Nafion films. Extraction of the intrinsic reaction entropy, ΔSᵣ꜀, yields a revised interpretation of the reaction entropy behavior of Ru complexes in Nafion films. The pH vs E_(1/2) properties of Nafion-bound Ru-(NH₃)₅(H₂O)^(3+/2+), Ru(NH₃)₅(isna)^(3+/2+) (isna = isonicotinic acid), and Ru(NH₃)₅(pz)^(3+/2+) (pz = pyrazine) have been investigated, and apparent p£a values have been determined for these complexes. Apparent pKₐ values in Nafion are generally higher than those in aqueous media by 1.5-2 pK units.

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