Alkali metal cation effects in a prussian blue surface-modified electrode
Graphite electrodes were surface modified first by coating them in a glow discharge with iron pentacarbonyl (the resulting deposit called an iron-containing plasma deposit) or iron pentacarbonyl and ethane in a 1:1 molar ratio (the resulting deposit called an iron-containing plasma polymer) and then further surface modified by electrochemical reaction with hexacyanoferrate ion in water. This results in a Prussian blue surface modified electrode, which was characterized by cyclic voltammetry in dry propylene carbonate and in 1 vol % water/propylene carbonate containing K+, Na+, or Li+ electrolyte cations. Depending on the potential of the electrode, the surface adherent iron hexacyanoferrate is present as Berlin green, [KFe3+FeIII (CN)6]1/2[Fe3+FeII(CN)6]1/2, Ĭsolubleĭ Prussian blue, KFe3+FII(CN)6, or Everitt's salt, K2Fe2+FeII(CN)6, when K+ is the cation of the bulk electrolyte. When Na+ or Li+ salts are used as the background electrolyte, the electrode surface bound iron hexacyanoferrate changes from the totally oxidized species Fe3+FeIII(CN)6 to Everitt's salt, M2Fe2+FeII(CN)6, with one or two intermediate redox processes. The redox potentials for these processes are influenced by the alkali-metal countercation. The dependence of peak currents on scan rates suggests that the electrode redox processes are kinetically controlled by electrolyte cation migration in and out of the surface-bound Prussian blue lattice. Adherence of the surface-bound Prussian blue is remarkable in a 1 vol % water/propylene carbonate solution. No decrease in peak current signals was observed after 9 days of continuous redox cycling using an iron-containing plasma deposit Prussian blue surface modified electrode. A unique characteristic of this Prussian blue surface modified electrode is that it is permeable to K+, Na+, and Li+ cations and exhibits a selectivity sequence in dry propylene carbonate or a 1 vol % water/propylene carbonate solution in the order K+ > Na+ > Li+. It is presumed that these alkali-metal countercations occupy interstitial cavities in the cubic Prussian blue lattice. Results are presented which suggest that the hydrated cation enters these cavities and not the naked cation when the solvent is 1 vol % water/propylene carbonate or 100% water. However, when the solvent is dry propylene carbonate, the naked cation enters the Prussian blue lattice and not the solvated cation. Results obtained from experiments using the Li+ cation in propylene carbonate and water/propylene carbonate mixtures suggest that the countercation lattice sites associated with the low-spin Fe(III/II) redox couple are chemically distinguishable from those associated with the high-spin Fe(3+/2+) redox couple. © 1984, American Chemical Society. All rights reserved.
Crumblis, AL; Lug, PS; Morosoff, N
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