The Working Electrode Surface

Polarisation titrations are often referred to as amper-ometric or biamperometric titrations. It is necessary that one of the substances involved in the titration reaction be oxidisable or reducible at the working electrode surface. In general, the polarisation titration method is applicable to oxidation-reduction, precipitation and complex-ation titrations. Relatively few applications involving acid/base titration are found. Amperometric titrations can be applied in the determination of analyte solutions as low as ICE5 M to 10-6 M in concentration. 

Unsteady-state effects and transition times are also significant in forced convection. Whenever the mass-transfer boundary layer is large anywhere on the working electrode surface, the commonly employed experimental time range of a few minutes may not be adequate to reach the steady-state limiting current. 

In principle the ISO-NOP sensor works as follows. The sensor is immersed in a solution containing NO and a positive potential of —860 mV (vs Ag/AgCl reference electrode) is applied. NO diffuses across the gas permeable/NO-selective membrane and is oxidized at the working electrode surface producing a redox current. This oxidation proceeds via an electrochemical reaction followed by a chemical reaction. The electrochemical reaction is a one-electron transfer from the NO molecule to the electrode, resulting in the formation of the nitrosonium cation ... 

X is oxidized to a measurable extent at potentials more positive than Ei. Potentials lower than Ei apparently do not supply sufficient energy to activate oxidation of the molecules X that reach the working electrode surface (by diffusion and/or convection) during passage through the cell. Potentials increasing from Ei to E2 supply sufficient... 

Ei/2 50% of the molecules X that reach the working electrode surface are electrolysed. Ex/2 is a characteristic value for X (under the chromatographic conditions given) and can be very helpful for identification. 

Thin-layer cell design is based on reduction of diffusion path length the mobile phase is directed along the working electrode surface as a thin film of liquid (see Figure 3-1). 

Electrolytic efficiency (represented by analyte peak current) and noise as a function of the working electrode surface area is shown in Figure 3-7. 

The working electrode surface in contact with the mobile phase contributes to background current and thereby to noise. For optimum signal-to-noise ratio all wetted working electrode surfaces should therefore contribute to analyte electrolysis as well. 

The internal surfaces of pores in the working electrode surface are wetted but do not contribute to electrolysis of analyte (transport of analyte into pores takes too much time). Therefore working electrode surfaces preferably should be of high density and well polished (relevant for thin layer and wall-jet design only). 

The Hewlett-Packard HP 1049A Electrochemical Detector cell has a unique system for working electrode replacement the working electrode itself (an 8 mm diameter disc) can be removed from its holder, allowing for economic working electrode replacement in case the working electrode surface cannot be restored. 

In potentiostatic exhaustive electrolysis, the potential of the working electrode is constant throughout the experiment. The substrate is transported by convection and diffusion to the working electrode surface. The current decreases with the bulk concentration of the substrate, if no further electron transfers occur. The charge transferred is... 

The enhancement of SWV net peak current caused by the reactant adsorption on the working electrode surface was utilized for detection of chloride, bromide and iodide induced adsorption of bismuth(III), cadmium(II) and lead(II) ions on mercury electrodes [236-243]. An example is shown in Fig. 3.13. The SWV net peak currents of lead(II) ions in bromide media are enhanced in the range of bromide concentrations in which the nentral complex PbBr2 is formed in the solntion [239]. If the simple electrode reaction is electrochemically reversible, the net peak cnnent is independent of the composition of supporting electrolyte. So, its enhancement is an indication that one of the complex species is adsorbed at the electrode snrface. 

The current signal, on the other hand, can be reduced drastically. A first, obvious, method is reducing the working electrode surface. A second possibility involves reducing the amount of reacting electroactive component which is transported to the electrode surface, e.g. by means of a membrane or a porous wall. When the working electrode, the reference electrode and the counter electrode are positioned at the same side of the membrane, the resistance of the solution between the electrodes is not influenced by the membrane, which is not the case when the electrodes are positioned at different sides of the membrane, and the influence on the IR voltage drop can... 

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