Practical Problems in Voltammetry : 4 . Preparation of Working Electrodes

Abstract

chemical reactions is the transfer of electrons between the electrode surface and molecules in the interfacial region (either in solution or immobilized at the electrode surface). The kinetics of this heterogeneous process can be significantly affected by the microstructure and roughness of the electrode surface, the blocking of active sites on the electrode surface by adsorbed materials, and the nature of the functional groups (e.g., oxides) present on the surface (1, 2). Therefore, there has been considerable effort devoted to finding methods that remove adsorbed species from the electrode and produce an electrode surface that generates reproducible results. Some of these methods have also resulted in the activation of the electrode surface (as judged by an increase in the rate of electron transfer). These methods are the subject of this paper, and include mechanical polishing, heat pretreatment, and electrochemical pretreatment. The most common method for surface preparation is mechanical polishing. The protocol used for polishing depends on the application for which the electrode is being used and the state of the electrode surface. There are a variety of different materials available (e.g., diamond, alumina, silicon carbide), with different particle sizes suspended in solution (BAS supplies 0.05 μm alumina polish and 1, 3, 6, and 15 μm diamond polishes). The pad used for polishing also depends on the material being used for polishing—Texmet pads are used with alumina polish, and nylon pads should be used with diamond polish. Working electrodes supplied by BAS have first been lapped to produce a flat surface, and have then been extensively polished to a smooth, mirror-like finish at the factory. Therefore, they typically only require repolishing with 0.05 μm or 1 μm diamond polish by the user in between experiments. Materials that have a rougher surface (e.g., electrodes which have been scratched) must first be polished using a larger-particle polish in order to remove the surface defects. After the defects have been removed, the polishing should continue with successively smaller-particle-size polish (e.g., 15 μm, then 6 μm, then 3 μm, and then 1 μm). Once polishing has been completed (this can require from 30 s to several minutes, depending upon the state of the electrode), the electrode surface must be rinsed thoroughly with an appropriate solvent to remove all traces of the polishing material (since its presence can affect the electron transfer kinetics). Alumina polishes should be rinsed with distilled water and diamond polishes with methanol or ethanol. The rinsing solution should be sprayed directly onto the electrode surface. After the surface has been rinsed, electrodes polished with alumina should also be sonicated in distilled water for a few minutes to ensure complete removal of the alumina particles. If more than one type of polish is used, then the electrode surface should be thoroughly rinsed between the different polishes. As discussed above, the effect of any surface pretreatment can be determined by its effect on the rate of electron transfer. This can be judged qualitatively by examining the separation of the peak potentials in a cyclic voltammogram of a molecule whose electron transfer kinetics are known to be sensitive to the state of the surface; a more quantitative determination can be made by calculating the value of ks from this peak potential separation. For example, ks for potassium ferricyanide at glassy carbon surface following a simple polishing protocol was found to lie in the range 0.01 0.001 cm s-1 (3,4) (this should be compared with the values measured for ks for a platinum electrode, which are at least one order of magnitude larger). The strong dependence of the electron transfer kinetics of ferricyanide on the state of the electrode surface means that Practical Problems in Voltammetry: 4. Preparation of Working Electrodes

Cite this paper

@inproceedings{Bott2000PracticalPI, title={Practical Problems in Voltammetry : 4 . Preparation of Working Electrodes}, author={Adrian W. Bott}, year={2000} }