The working electrode W

It is often recommended by manufacturers or others that working electrodes be polished prior to the measurements. We cannot recommend this as a general procedure. In our experience platinum electrodes, for example, improve in quality by being used and, after a measurement or a series of measurements, they should only be rinsed by an organic solvent and wiped to dryness by a soft paper tissue. Only when this gentle treatment is not sufficient to remove material from the surface, for instance a deposit, should the electrode be re-polished. 

For reductions, hanging mercury drop electrodes or mercuryfilm electrodes are frequently used owing to their microscopic smoothness and because of the large overpotential for hydrogen evolution characteristic for this electrode material. Mercury film electrodes are conveniently prepared by the electrochemical deposition of mercury on a platinum electrode from an acidic solution of an Hg2+ salt, e.g. the nitrate. However, the oxidation of mercury metal to mercury salts or organomercurials at a low potential, 0.3-0.4 V versus the saturated calomel electrode (SCE), prevents the use of these electrodes for oxidations. 

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The electrical characteristics of the cell and electrode will comprise both capacitative and resistive components, but for simplicity the former may be neglected and the system can be represented by resistances in series (Fig. 19.36 > and c). The resistance simulates the effective series resistance of the auxiliary electrode A.E. and cell solution, whilst the potential developed across by the flow of current between the working electrode W.E. and A.E. simulates the controlled potential W.E. with respect to R.E. 

In this technique the working electrode W (e.g. Pt) is exposed to the reactive gas mixture (e.g. C2H4 plus O2) and also serves as the catalyst for a catalytic reaction, e.g. ... 

Fig. 1.2. Flow cell for adsorption measurements and on-line mass spectroscopy. The working electrode (w.e.) at the bottom of the cell is conneced directly to the MS c.e. = counter electrode, r.e. = reference electrode.

Any container or a flow system with three electrodes closely placed can be used for electrochemical studies. Some electrochemical cells are shown in Fig. 18b. 1. Most electrochemical cells contain three electrodes. These are the working electrode (W), counter electrode (C), and the reference electrode (R). Table 18b.2 shows the materials and properties of W, R, and C. 

Usually, the working electrode (W) is a porous metallic electrode in PEVD. Thus, reactant (B) in the vapor phase can reach the surface of the solid electrolyte for initial electrochemical reaction at a three-phase boundary of solid electrolyte (E), working electrode (W) and sink vapor phase (S) as shown in Eigure 3 (location II). All reactants for the sink side electrochemical reaction (1) or (2) are only available there. Subsequent reaction and deposition of the product (D) requires both electrons and ions to travel through product (D) to the surface to react with vapor phase reactant(s) electrochemically at location III in Eigure 3. 

Once the working electrode (W) is covered by the ionic conducting product or the entire solid electrolyte (E) is covered by the electronic conducting product, no electrically shorted surface exists. Thus, further growth in thickness has to involve diffusion of both the ionic species and electrons to the surface to react with the gas phase. Practically, diffusion of one species is much faster than the other. However, electroneutrality must be maintained under this open circuit condition. The growth rate is determined by either migration of electrons or mobile ionic reactants in the deposit (D). In both cases, the increase in thickness should follow the parabolic law. ... 

Fig. 1C Schematic representation of a three-electrode circuit, showing the working electrode, (W.E.), reference electrode, (R.E.) and counter electrode C.E.).

When a current I flows between the working electrode (W) and the counterelectrode (C), then the potential difference between them, (./ vc deviates from its open-circuit value U c (Fig. 14). The electrochemical cell overpotential rjsfjc is then... 

A basic circuit is shown schematically in Fig. 19.36(a). The specimen C., or working electrode W.E. is the metal under study, the auxiliary electrode A.E. is usually platinum and R.E. is the reference electrode, for instance a saturated calomel electrode. The desired potential difference between the specimen and the reference electrode is set with the backing circuit B. Any... 

Fig. 19.36 Basic circuit for a poiemiostat. (a) Basic circuit for a potentiostat and electrochemical cell, (b) Equivalent circuit, (c) Circuit of a basic potentiostat. A.E. is the auxiliary electrode, R.E. the reference electrode and W.E. the working electrode (6 and c are from Polen-tiostat and its Applications by J. A. von Fraunhofer and C. H. Banks, Butlerworths (1972))...

When a solid electrolyte component is interfaced with two electronically conducting (e.g. metal) films (electrodes) a solid electrolyte galvanic cell is formed (Fig. 3.3). Cells of this type with YSZ solid electrolyte are used as oxygen sensors.8 The potential difference U R that develops spontaneously between the two electrodes (W and R designate working and reference electrode, respectively) is given by ... 

Figure 5.19. The physical origin of NEMCA When a metal counter electrode (C) is used in conjunction with a galvanostat (G) to supply or remove ions [O2 for the doped Zr02 (a), Na+ for P"-A1203 (b)] to or from the polarizable solid electrolyte/catalyst (or working electrode, W) interface, backspillover ions [O6 in (a), Na5+ in (b)] together with their compensating charge in the metal are produced or consumed at the tpb between the three phases solid electrolyte/catalyst/gas. This causes an increase (right) or decrease (left) in the work function of the gas-exposed catalyst surface. In all cases AO = eAUWR where AUWr is the overpotential measured between the catalyst and the reference electrode (R).

In the EMIRS (and in situ FTIR) technique, the potential of the working electrode is changed from a base value, Vb, at which the reflectivity of the electrode is R(v)b, to a value Kw, where the reflectivity is R(v)w. Spectra are usually plotted in the form (AR/R) vs, v, where ... 

Most common reference electrodes are silver-silver chloride (SSC), and saturated calomel electrode (SSC, which contains mercury). The reference electrode should be placed near the working electrode so that the W-potential is accurately referred to the reference electrode. These reference electrodes contain concentrated NaCl or KC1 solution as the inner electrolyte to maintain a constant composition. Errors in electrode potentials are due to the loss of electrolytes or the plugging of the porous junction at the tip of the reference electrode. Most problems in practical voltammetry arise from poor reference electrodes. To work with non-aqueous solvents such as acetonitrile, dimethylsulfoxide, propylene carbonate, etc., the half-cell, Ag (s)/AgC104 (0.1M) in solvent//, is used. There are situations where a conventional reference electrode is not usable, then a silver wire can be used as a pseudo-reference electrode. 

Fig. 10.9 Schematic representation of a three electrode cell. W - working electrode R -reference electrode and S - subsidiary electrode. Note that the reference electrode is placed as close as possible to the working electrode.

Figure 3 Schematic showing three-electrode DC operation. C refers to the counter electrode, W refers to the working electrode. The reference electrode can either be on the counter side (Rl) or the working side (R2).

A typical example would be a coulometric controlled-potential experiment, where initially a large current exists between the auxiliary and the working electrode (the load of the control amplifier). If a potentiostat is rated to have a maximum output of 20 V at 1 A, it cannot supply more than 20 W of power [power (watts) = current (amperes) x potential (volts)]. If Rt were 100 Q, the potentiostat would not be able to control the potential of the working electrode at -2.0 V (or any other potential for that matter) if 0.5 A were demanded. At least 25 W (I2Rt) of power would be required of the potentiostat for potential control to be maintained. As a result of our 5-W deficiency, the potential of the working electrode would be uncontrolled at a value less than the -2.0 V less than 0.5 A, in fact, would pass through the cell. 

A schematic diagram of a basic electrochemical cell is shown in the insert of Fig. 5.1. It contains a working electrode W, auxiliary electrode AUX, signal source S (current or voltage), and meter M for measurement of voltage or current. The ionic medium in which the electrodes are immersed is an electrolyte, which can be either liquid or solid. 

At first, no current flows through the cell until a decomposition potential Ed- is reached. At that point, the current begins to flow (Curve A). We also observe that gas bubbles are formed at the working electrode, and that current fluctuates somewhat randomly. Two chemical processes are taking place at the electrodes. At the W electrode (which is now the cathode), electrons are transferred from the electrode to the hydrogen ions, H+. Thus, the reduction takes place at the cathode, according to the following electrochemical reaction. 

NADH can be readily monitored electrochemically, and can be used as a simple and effective method to monitor metal ion concentrations. Such an approach has been recently utilised by Rodriguez et al. [149] for an SPCE-based biosensor for the amperometric detection of Hg2+, Cu2+, Cd2+, Zn+ and Pb2+. Devices used in this study were printed onto 250 pm thick polyester sheet. The working electrode (planar area 0.16 cm2) was fabricated from a commercially available carbon powder containing 5% rhodium plus promoters, which was made into a screen-printable paste by mixing 1 4 in 2.5% (w/v) hydroxyethyl cellulose in water. The reference electrode ink contained 15% silver chloride in silver paste. The counter electrode and basal tracks were fabricated... 

When PLD is considered together with the solution resistance Rsoh between reference electrode (R.E.) and working electrode (W.E.), the total current Itot(t) is written as... 

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