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Extra info for A Chemical & Theoretical Way To Look At Bonding On Surfaces Hoffmann [Article]

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1 M TBAPF6 at different scan rates. Measurements taken with a Pt microdisk electrode (r0 ϭ 6 ␮m). 40 J. Heinze and K. Borgwarth In the stationary state, the currents are time-independent and the scan no longer affects the shape and size of the voltammograms. The conditions for the formation of a steady-state situation differ according to the geometry of the electrode used. 49–54 The only exception is the case of the reversible redox process during spherical diffusion to a spherical electrode. For this, Reinmuth55 has developed a correction formula that can be used to calculate the dimensionless current ␲1/2 ϫ␹s(at) (which corresponds to a normalized current – S stands for spherical) from the known current ␲1/2 ϫ␹p(at) (p stands for planar) for planar diffusion (Eqn (16a) where ␴ is the sphericity (16b) and ⌽ the spherical correction factor (16c)).

Camman, Sensors Actuators A 43 (1994) 296 Principles of EMST 31 55. H. Meyer, B. Naendorf, M. Wittkampf, B. Gründig, K. Cammann, R. Kakerow, Y. Manoli, W. Mokwa, M. Rospert, in: A. van den Berg, P. Bergveld (eds), Micro Total Analysis Systems, Kluwer, 1995, p. 245 56. D. Schattka, S. Winkels, J. W. Schultze, Metalloberfläche, 1997, p. 51 57. M. Datta, in: M. Datta, K. Sheppard, D. Snyder (eds), Electrochemical Microfabrication, The Electrochemical Society Proceedings, Pennington, New York, 1992, p.

As it is obvious that several parameters determine the specific properties of such electrodes it is very difficult to define the term ‘ultramicroelectrode’ in terms of a precise limit for its characteristic dimension. In the literature it has become common practice to use the term ‘UME’ for disk electrodes with a diameter of 20 ␮m or less. 47,48 As one might expect, the influence of electrode shape is greatest in the case of spherical diffusion fields. 3 shows that already after 1 s, despite a small absolute current, the current density at a spherical microelectrode with a diameter of 1 ␮m is nearly 100 times greater than that at a normal electrode (Eqn (15)): ΂ ΃ ispha¨r Dt ϭ1ϩ ␲ 2 iplan r0 1/2 (15) The calculations for cylindrical, band and ring electrodes, prove that these attain only a quasi-stationary state, as the equations for their currents contain Homogeneous reaction kinetics 39 time-dependent terms.

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