Electrochemical Cells for External Reflection

The cell design is particularly difficult, due to the fact that the solvent strongly absorbs the infrared radiation in aqueous electrolyte, for instance, it is hardly possible to work with a solution thickness greater than a few microns, which practically imposes the use of flat, smooth (perfectly polished) electrodes. Bewick has examined in detail the problem of the absorption of the infrared radiation at the electrode/aqueous solution interface. Two types of estimations were made for wavelengths close to the O—H stretching vibration bands—firstly, by the direct application of Beer s law and, secondly, by a rigorous application of the Fresnel equations. Fortunately, it appears that the absorption of the infrared radiation is much less for the solution and much more for the adsorbed monolayer than that which is predicted, in first approximation, by Beer s law. 

The solubility is really a critical point due to two factors. Firstly, a more or less long contact of the window with the solution can depolish it. Secondly, the metallic ions formed by dissolution can be electrochemically redeposited on the electrode surface and can thus modify its adsorption properties. 

Data taken from ORIEL, Eigenschaften optischer Materialen, except where otherwise noted. 

FIGURE 21. Plots of transmission T versus wavelength in the infrared range for various materials. After ORIEL, Eigenschaften optischer Materialen, Darmstadt, FRG (1981). Thickness given in millimeters. 

FIGURE 22. Schematic diagram of the electrochemical cell used for external reflection. 

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