Anal. Chem. 2007, 79, 291-298
Thermal Modulation Voltammetry with Laser Heating at an Aqueous|Nitrobenzene Solution Microinterface: Determination of the Standard Entropy Changes of Transfer for Tetraalkylammonium Ions Teruo Hinoue,* Eiji Ikeda, Shigeru Watariguchi, and Yasuyuki Kibune
Department of Chemistry, Faculty of Science, Shinshu University, Matsumoto, Nagano 390-8621, Japan
Thermal modulation voltammetry (TMV) with laser heating was successfully performed at an aqueous|nitrobenzene (NB) solution microinterface, by taking advantage of the fact that laser light with a wavelength of 325.0 nm is optically transparent to the aqueous solution but opaque to the NB solution. When the laser beam impinges upon the interface from the aqueous solution side, a temperature is raised around the interface through the thermal diffusion subsequent to the light-to-heat conversion following the optical absorption by the NB solution near the interface. Based on such a principle, we achieved a fluctuating temperature perturbation around the interface for TMV by periodically irradiating the interface with the laser beam. On the other hand, the fluctuating temperature perturbation has influence on currents for transfer of an ion across the interface to produce fluctuating currents synchronized with the perturbation through temperature coefficients of several variables concerning the transfer, such as the standard transfer potential and the diffusion coefficient of the ion. Consequently, TMV has the possibility of providing information about the standard entropy change of transfer corresponding to a temperature coefficient of the standard transfer potential and a temperature coefficient of the diffusion coefficient. In this work, the aqueous|NB solution interface of 30 µm in diameter was irradiated with the laser beam at 10 Hz, and the currents synchronized with the periodical irradiation were recorded as a function of the potential difference across the interface in order to construct a TM voltammogram. TM voltammograms were measured for transfer of tetramethylammonium, tetraethylammonium, tetrapropylammonium, and tetra-n-butylammonium ions from the aqueous solution to the NB solution, and the standard entropy change of transfer was determined for each ion, according to an analytical procedure based on a mathematical expression of the TM voltammogram. Comparison of the values obtained in this work with the literature values has proved that TMV with laser heating is available * To whom correspondence should be addressed. E-mail: gipac.shinshu-u.ac.jp. 10.1021/ac061315l CCC: $37.00 Published on Web 12/02/2006
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© 2007 American Chemical Society
for the determination of the standard entropy change of transfer for an ion. Since every electrode reaction is thermally influenced through temperature coefficients of many variables concerning elemental electrode processes in the electrode reaction, electrochemical methods with a few kinds of thermal perturbations, e.g., a fluctuating temperature modulation and a temperature jump, have been proposed to elucidate the electrode reaction.1-6 Among the methods, thermal modulation voltammetry (TMV), which was first proposed by Miller,5 is a convenient method, and thus one can easily achieve the instrumentation with a heat source for periodic heating of the electrode and a lock-in amplifier for synchronous detection of the current response other than conventional instruments for linear sweep voltammetry, such as a potentiostat, a potential sweeper, and an X-Y recorder. In TMV by Miller,5,7 a thin gold film attached to a steel tube, serving as a rotating disk electrode, was periodically irradiated with an argon ion laser beam from the film back in order to modulate a temperature at the electrode-electrolyte interface. Further, Miller and Valdes theoretically elucidated heat and mass transports under hydrodynamic convection at the rotating disk electrode and investigated the current response to the thermal modulation.8-11 Hinoue et al. performed TMV with laser heating, where an intermittent argon ion laser beam impinged upon a fine platinum electrode, being embedded in a bottom wall of a thin-layer flow electrolytic cell, through an electrolyte solution flowing in the cell, and confirmed that the current responses to the thermal modulation for (1) Barker, G. C.; Gardner, A. W. J. Electroanal. Chem. 1975, 65, 95-100. (2) Harima, Y.; Aoyagi, S. J. Electroanal. Chem. 1976, 69, 419-422. (3) Harima, Y.; Aoyagi, S. J. Electroanal. Chem. 1977, 81, 47-52. (4) Benderskii, V. A.; Velichko, G. I. J. Electroanal. Chem. 1982, 140, 1-22. (5) Miller, B. J. Electrochem. Soc. 1983, 130, 1639-1640. (6) Smalley, J. F.; Krishnan, C. V.; Goldman, M.; Feldberg, S. W. J. Electroanal. Chem. 1988, 248, 255-282. (7) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, Fundamentals and Applications; 2nd ed.; Wiley: New York, 2001; pp 359-360. (8) Valdes, J. L.; Miller, B. J. Phys. Chem. 1988, 92, 525-532. (9) Valdes, J. L.; Miller, B. J. Phys. Chem. 1988, 92, 4483-4490. (10) Valdes, J. L.; Miller, B. J. Electrochem. Soc., 1988, 135, 2223-2231. (11) Valdes, J. L.; Miller, B. J. Phys. Chem. 1989, 93, 7275-7280.
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[Fe(CN)6]3-/4- and Fe2+/3+ are opposite in sign, depending on the sign of the standard entropy change of the electrode reaction.12 As mentioned first, the electrode reaction is thermally influenced through temperature coefficients of many variables concerning the mass transport, the thermodynamics, and the kinetics. However, when the electrode reaction is reversible, the thermal modulation with a relatively small temperature amplitude can only influence the limiting current (the mass transport) and the standard electrode potential (the thermodynamics).11 This fact therefore suggests that the current response obtained from TMV is available for the determination of the standard entropy change of the electrode reaction, ∆S°, because the thermodynamic relation, ∆S° ) nF(∂E°/∂T)P for Ox + ne- a Red, indicates that ∆S° corresponds to the temperature coefficient of the standard electrode potential, E°, at a constant pressure, P. Here, F and T stand for the Faraday constant and the temperature. While the above thermodynamic relation suggests that ∆S° is basically determined from electromotive force measurements of an electrochemical cell containing a redox couple of interest at different temperatures, the measurements require a nonisothermal galvanic cell, and thus, a thermal diffusion potential resulting from the Soret effect is inevitably generated, which must be corrected in order to determine ∆S° accurately.8,13,14 However, in contrast to the electromotive force measurement, TMV is hardly influenced by the Soret effect, because TMV usually employs the thermal modulation with a small temperature amplitude (
