Pretreatment electrode

Electrode Pretreatment. There is ample evidence that the rate of electron transfer at a solid electrode is sensitive to the surface state and previous history of the electrode. An electrode surface that is not clean usually will manifest itself in a voltage-sweep experiment to give a decrease in the peak current and a shift in the peak potential. Various pretreatment methods have been employed to clean or activate the surface of electrodes the process is intended to produce an enhancement of the reversibility of the reaction (i.e., produce a greater rate of electron transfer).97 This activation or cleaning process may function in two ways by removing adsorbed materials that inhibit electron transfer and by altering the microstructure of the electrode surface. 

Sometimes the same result can be attained by heavily anodizing (e.g., 1 s at 10-100 mA cm-2) the electrode surface to remove adsorbed material. The oxide layer then is reduced at a potential sufficient to remove the oxide layer but not reduce hydronium ion or solvent. The pretreatment cycle is repeated before every experiment, usually in the test solution. For voltammetric work in aqueous solution, chemical pretreatment and electrochemical cycling is preferred to simple polishing or in addition to polishing. 

Construction and Mass-Transport Properties of Voltammetric Electrodes 

Mercury electrodes. Mercury is a widely used electrode material for the study of cathodic processes because of its high overpotential for hydronium 

The presence of noble metals in mercury will lead to a reduced overpotential for the evolution of hydrogen and should be reduced to trace levels ( 0.1 ppm). This is usually accomplished by vacuum distillation or by distillation in the presence of a stream of air. Base metals should be removed by chemical treatment before distillation, and the still that is employed should be reserved for purifying mercury for electrochemical use. 

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A general problem existing with all multicomponent catalysts is the fact that their catalytic activity depends not on the component ratio in the bulk of the electrode but on that in the surface layer, which owing to the preferential dissolution of certain components, may vary in time or as a result of certain electrode pretreatments. The same holds for the phase composition of the surface layer, which may well be different from that in the bulk alloy. It is for this reason that numerous attempts at correlating the catalytic activities of alloys and other binary systems with their bulk properties proved futile. 

For the Pt(llO) electrode, there are some contradictory results regarding its catalytic performance compared with Pt(lOO) some studies indicate that the activity is higher for Pt(llO), whereas others suggest the opposite [Chang et al., 1990 Clavilier et al., 1981 Lamy et al., 1983]. The differences are probably associated with different surface states of the Pt(l 10) electrode. The acmal surface strucmre of the Pt(llO) electrode is strongly dependent on the electrode pretreatment. Since formic acid oxidation is a surface-sensitive reaction, different electrocatalytic behavior can be obtained for the same electrode after different treatments. 

A platinum electrode pretreated in the way as is described in Section 1.2 may show some minimal desorption of carbonaceous residues which may come from C-atoms diffusing from the bulk of Pt or from the rest of the gas in the UHV. A blank desorption experiment carried out by transferring a Pt electrode which was held at 450 mV in H2S04 for 120 s is shown in Fig. 2.4(a). 

Figure 23. In situ X-ray absorption spectrum for half a monolayer of copper underpotentially deposited on a bulk Pt (111) electrode pretreated with iodine.

Sipos et al. [789] have described a procedure for the simultaneous determination of copper and mercury in seawater down to the ng/1 range using differential pulse ASV at a gold electrode. Pretreatment is necessary, and comprises UV irradiation to release the trace metal bound to dissolved organic matter. 

Repeated visual examination and impedence measurements indicated no degradation of the electrode over the course of the experiments (3 weeks). During this time, the selected electrodes were stored in the inert-atmosphere glove box in which the electrochemical experiments were performed. No electrode pretreatment procedures were used and the crystals were not recleaved. 

Figure 3 shows the cyclic voltammograms of cytochrome c recorded in aqueous solution either when the protein is adsorbed (or immobilized) on the electrode (Sn02) surface (a), or when it diffuses to a gold electrode pretreated by adsorption of 4,4 -bipyridyl (b). 

Figure 3 Cyclic voltammograms recorded at different scan rates in aqueous solution (pH 7) of cytochrome c, under the following experimental conditions (a) protein adsorbed on the Sn02 electrode surface (b) Au electrode pretreated with bipyridyl protein in solution...

Figure 11 Cyclic voltammogram of cytochrome b5 recorded at a gold electrode pretreated with Cys-Lys-Cys. Aqueous solution at pH 7.0 (KCl 0.1 mol dm 3). Scan rate 0.005 Vs 1...

Figure 37 Cyclic voltammogram recorded at a carbon paste electrode pretreated with 4,4 -bipyridyl in an aqueous solution (pH 2.2) of rusticyanin...

Figure 40 Cyclic voltammograms recorded at a carbon paste electrode pretreated with 4,4-bipyridyl in an aqueous solution of azurin (from Pseudomonas Aeruginosa,) buffered at pH 7.0. Scans at different times (i) after 4 min (ii) after 13 min (iii) after 85 min...

The changes in reorientation of surface atoms were explained using the dynamic model of the crystal space lattice. It was assumed that during anodic polarization, when the oxidation of adsorbed water is taking place, atoms oscillate mainly in a direction perpendicular to the electrode surface. This process leads to periodic separation of atoms in the first surface layer. Thus, the location of atoms in different orientations is possible. It was stated that various techniques of electrode pretreatment used for... 

They found that a Cu electrode, pretreated by immersing it in a 0.1M BTA solution for 15 seconds, inhibited the 0 reduction reaction initially and that on subsequent cycles the currents Increased to that of bare Cu in a short time. A similar effect was observed when a Cu electrode was cycled in a ImM solution of BTA. They discovered that a solution of 0.1M BTA produced a lasting effect, indicating that a reservoir of BTA is necessary for continuous protection of the copper against corrosion. We found that bare Cu gives the same voltammogram in the 0 reduction region in both acetate buffer and phosphate buffer therefore, McCrory-Joy et. al. s results can be directly compared to the results reported here. 

Again we refer to the review by Tallec [36] where the effect of system parameters on enantioselectivity is summarized. For a given substrate, the most important factors are electrode material, electrode pretreatments, modifier structure, solvent, electrolyte, pH and buffer system, voltage and temperature. 

Far from the metal trace analysis, our initial studies with BCFMEs were focused on the determination of folic acid [122], In this case, the main goal was the optimisation of the electrode pretreatment for this analyte. An acidic medium (0.1M perchloric acid) was considered optimum for folic acid determination by differential pulse voltammetry. A linear range between 2.0 x HT8 and 1.0 x 10 6M with a detection limit of 1.0 x 10 8M was obtained. Nevertheless, in this work, the adsorptive properties of the folic acid on mercury were noted and the employment of mercury-coated carbon fibre UMEs for folic acid determination has been targeted as a future goal. 

Fig. 4 Influence of the potential applied to the working electrode during the immobilization step on the guanine peak area. Electrode pretreatment + 1.6 V vs Ag/AgCl. ss-DNA immobilization 5 mg L-1 for 2 min chronopotentiometry conditions 2 x SSC buffer pH 7.4 with a stripping constant current of + 2 jxA and an initial potential of + 0.5 V...

Attaching the enzyme directly on the electrode surface is expected to improve elec-trocatalytic efficiency and response and improve the reproducibility of immobilization (147). Metallic (122, 144, 145) and carbonaceous (146) enzyme electrodes develop potentiometric responses to H2O2 produced by the enzymatic reaction. Unfortunately, the signal is markedly dependent on the redox surface of the electrode and thus on the electrode pretreatments (which are quite difficult to reproduce). 

The Eh-pH and Eh-Es2- relations in the H2S-H2O system. The Eh values in the H2S-H20 system were found to be slightly dependent upon stirring ( 10 mV]. At 298 K the potentials of the platinum electrodes were established quite slowly [1 to 2 hr] at pH > 5. At pH < 5 it generally takes less than one hour. The potentials were independent of such electrode pretreatment as cathodic or anodic polarization. In Figure 4 we present the Eh-pH relations obtained in the H2S-H20-NaCl(0.7 j4] system. 

The specifics of the degradation phenomena define the electrode pretreatment. In the case of mechanical treatment, the surface composition can vary due to heating [60], and this can be avoided by polishing in nonaqueous solutions [47] or by slowly cutting the surface layer while it is out of contact with air. Chemical treatment [60,61] will be discussed below in the context of the HTSC etching problem. 

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