Reflection Measurement at Special Electrode Configurations

The electrode substrate for the reflection measurement is not necessarily in the form of a flat plate in a quiescent electrolyte solution but sometimes in the form of a sphere or in a hydrodynamic condition. We here consider three typical examples of in-situ UV-visible reflectance measurement in special electrode configurations. 

The first example is the measurement at a mercury (Hg) electrode. After the invention of polarography, an enormous body of work on the adsorption of organic molecules has been made at a mercury electrode surface. However, mercury is a liquid metal and its hanging drop changes its shape in response to the change of the surface tension, thus to the electrode potential. It is a difficult task to measure the reflection change at a mercury drop electrode surface, since exclusion of the perturbation due to the change of the shape of the electrode is critical. 

So far, five types of mercury electrode have been used in specular reflection measurements (i) hanging mercury drop electrode (HMDE) [38, 39], (ii) mercury film deposited on a platinum or gold substrate [40-42], (iii) mercury pool electrode with or without an amalgamated platinum ring guide [43, 44], (iv) mercury drop bottom electrode placed on the optical window [45—47], and (v) mercury drop bottom electrode placed on an underlying ion-conductive optically transparent polymer film [48]. To avoid the difficulty due to mechanical vibration and shape change, the mercury drop bottom electrode would be useful. 

Barker tried to measure the ER signal using an HMDE, though the experimental performance using the HMDE was not satisfactory [38]. Even when the mercury drop bottom electrode placed on the optical window is used for IR reflection measurement by Blackwood and coworkers, the potential control at the bottom surface/electrolyte solution thin layer seems to fail because of too high uncompensated resistance required to control the potential [46, 47]. 

The use of a mercury drop bottom electrode placed on an underlying Nation fihn in ER measurement was demonstrated for the reaction of heptyl viologen incorporated in a Nafion film (Fig. 2.13) [48], A mercury drop was placed on a Nation 117 tihn of thickness 0.175 mm. The transparent nature of the Nafion as weU as the high conductivity for cations through its film made it actually possible to measure the ER spectrum of the redox reaction of heptyl viologen with a perpendicular incidence of the hght to the mercury electrode surface through the cell bottom window and the Nafion fihn. 

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