Veronika Zinovyeva - Electrochemical and Spectral Properties of Ferrocene (Fc) in Ionic Liquid: 1-Butyl-3-methylimidazolium Triflimide, [BMIM][NTf2]. Concentration Effects

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      Vorotyntsev   Mikhail A., Zinovyeva Veronika A., Konev Dmitry V., Picquet   Michel, Gaillon Laurent, Rizzi Cécile

      J. Phys. Chem. B 2009,  113,  1085–1099


      Several earlier studies of the electrochemical oxidation of   ferrocene (Fc) in room-temperature ionic liquids revealed an   essentially nonlinear dependence of the oxidation current on the   Fc concentration in its relatively dilute solutions, with its   formally calculated diffusion coefficient strongly increasing   with the concentration. Since no plausible mechanism leading to   this very unusual finding had been proposed, our study of Fc   solutions in 1-butyl-3-methylimidazolium triflimide,   [BMIM][NTf2], was performed to verify whether the   above observation originated from an incorrect determination of   the dissolved Fc concentration. Our observations have   demonstrated that reliable control of the Fc concentration in   solution is complicated by factors such as the low amount of Fc   used to prepare small-volume solutions or the great difficulty to   dissolve completely a solid powder in a solvent with an extremely   high viscosity. An unexpected additional complication is related   to a sufficiently high volatility of Fc which manifests itself   even at room temperature and especially at elevated temperatures   or/and in the course of vacuum treatment of its solutions or its   solid powder. Parallel measurements of electrochemical responses   and UV−visible spectra for several series of Fc solutions of   various concentrations (prepared with the use of different   procedures) have shown a perfect parallelism between the peak   current and the intensity of the absorption band in the range of   360−550 nm, leading us to the conclusion of a linear relationship   between the oxidation current and the molecularly dissolved Fc   concentration. The relations of these measured characteristics   with the estimated Fc concentration in these solutions have   demonstrated a much greater dispersion (attributed to the   difficulty of a precise measurement of the latter) but without a   significant deviation from the linearity in general. This finding   has allowed us to estimate the diffusion coefficient of this   species: D = (1.7 ± 0.2) × 10−7  cm2/s. The extinction coefficients for the maximum of   the absorption band (at 440 nm) of Fc have been compared for a   series of solvents: [BMIM][NTf2], acetonitrile, THF,   heptane, CH2Cl2, ethanol, and toluene. A   simple method to estimate reliably the concentration of solute Fc   in ionic liquids based on spectroscopic measurements has been   proposed, owing to the proximity of Fc absorption properties for   a great variety of solvents.

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