Reference electrode for electrolytic

Reference electrode for electrolytic reduction, 52, 28 Resorcinol dimethyl ether, 50, 52... 

REDUCTIVE AMINATION WITH SODIUM CYANOBOROHYDRIDE N.N-DIMETHYLCY-CLOHEXYLAMINE, 52, 124 Reference electrode for electrolytic reduction, 52, 28 Resorcinol dimethyl ether, 50,52 Rhodium-on-alumina, catalyzed reduction of aromatic nuclei, 51, 105... 

Measuring electrodes for impressed current protection are robust reference electrodes (see Section 3.2 and Table 3-1) which are permanently exposed to seawater and remain unpolarized when a small control current is taken. The otherwise usual silver-silver chloride and calomel reference electrodes are used only for checking (see Section 16.7). All reference electrodes with electrolytes and diaphragms are unsuitable as long-term electrodes for potential-controlled rectifiers. Only metal-medium electrodes which have a sufficiently constant potential can be considered as measuring electrodes. The silver-silver chloride electrode has a potential that depends on the chloride content of the water [see Eq. (2-29)]. This potential deviation can usually be tolerated [3]. The most reliable electrodes are those of pure zinc [3]. They have a constant rest potential, are slightly polarizable and in case of film formation can be regenerated by an anodic current pulse. They last at least 5 years. 

Figure 4a. Electrochemical cells for microwave conductivity measurements. Cell above microwave conduit (1) electrochemical cell (plastic tube, placed on working electrode material), (2) counter-electrode, (3) reference electrode, (4) electrolyte, (5) space charge layer, (6) diffusion layer, (7) contact to working electrode, (8) waveguide.

The reference electrode-solid electrolyte interface must also be non-polarizable, so that rapid equilibration is established for the electrocatalytic charge-transfer reaction. Thus it is generally advisable to sinter the counter and reference electrodes at a temperature which is lower than that used for the catalyst film. Porous Pt and Ag films exposed to ambient air have been employed in most previous NEMCA studies.1,19... 

Examination of the behaviour of a dilute solution of the substrate at a small electrode is a preliminary step towards electrochemical transformation of an organic compound. The electrode potential is swept in a linear fashion and the current recorded. This experiment shows the potential range where the substrate is electroactive and information about the mechanism of the electrochemical process can be deduced from the shape of the voltammetric response curve [44]. Substrate concentrations of the order of 10 molar are used with electrodes of area 0.2 cm or less and a supporting electrolyte concentration around 0.1 molar. As the electrode potential is swept through the electroactive region, a current response of the order of microamperes is seen. The response rises and eventually reaches a maximum value. At such low substrate concentration, the rate of the surface electron transfer process eventually becomes limited by the rate of diffusion of substrate towards the electrode. The counter electrode is placed in the same reaction vessel. At these low concentrations, products formed at the counter electrode do not interfere with the working electrode process. The potential of the working electrode is controlled relative to a reference electrode. For most work, even in aprotic solvents, the reference electrode is the aqueous saturated calomel electrode. Quoted reaction potentials then include the liquid junction potential. A reference electrode, which uses the same solvent as the main electrochemical cell, is used when mechanistic conclusions are to be drawn from the experimental results. 

The most widely used working electrodes are vitreous carbon, platinum, gold and mercury (Fig. 19.1). These electrodes are flexible because they can be used between two potential values that depend on the support electrolyte, the pH and the nature of the reference electrode. For example, the limits for the Pt electrode are +0.65 V relative to the standard calomel reference electrode (SCE) (oxidation of water HzO —> 5O2 + 2H++ 2e ) and —0.45 V (reduction of water HzO + 2e H2 + 20H"). 

Reference Electrodes For Use in Fused-Salt Systems. The kinds of investigations that require reliable reference electrodes in molten salts duplicate those in aqueous electrolytes. Specifically, these include (1) thermodynamic... 

Electrodes of this type are sometimes called electrodes of the second class and are applied in electrochemistry as the reference electrodes for measuring unknown potentials. They are formed by a metal in contact with its insoluble salt, which is immersed into a solution of a soluble electrolyte with the... 

Another alternative, an ionized air reference electrode (IAE), has been described by Foulkes et al. [66] for use with solid polymer electrolytes, in particular Nafion 425. Use of the IAE is achieved by establishing electrical contact with the solid electrolyte from a weak source of ionizing radiation to ionize an air gap. In contrast to conventional reference electrodes, the IAE is a liquid-free device and can make electrical contact with a solid polymer electrolyte without a salt bridge. It is also temperature independent. In view of these special characteristics, the IAE might provide a useful alternative to conventional reference electrodes for use with solid polymer electrolytes. 

This anodic reaction provides sodium ions and electrons to the solid electrolyte and the inert Pt counter electrode, respectively, at the source side. Both the sodium ions and electrons will then travel through the solid electrochemical cell along previously-mentioned ionic and electronic paths to sustain the PEVD cathodic reaction for Na COj product formation at the sink side. Eurthermore, based on anodic reaction 60, the chemical potential of sodium is fixed by the vapor phase at the source side. Under open circuit conditions, this type of source can also serve as the reference electrode for a CO potentiometric sensor. 

The calomel electrode consists of mercury covered by a layer of calomel (Hg2Cl2), which is in contact with an electrolyte solution containing CT. Calomel electrodes are frequently used as reference electrodes for pH measurements using glass pH electrodes. 

Solid-state reference electrodes for potentiometric sensors are currently under research. The main problem to be faced in developing this type of electrode lies in connecting the ionic conducting (usually aqueous) solution with an electronic conductor. Since the reference electrode has to maintain a defined potential, the electrochemical reaction with components of the electrolyte has to be avoided. Oxides, mixed oxides, and polyoxometalate salts of transition elements can be proposed for preparing solid-state reference electrodes. Tested compounds include tungsten and molybdenum oxides (Guth et al., 2009). 

FIGURE 10.6 UETV, electrochemical device C, electrochemical cell, with sample SC, auxiliary and reference electrodes E, electrolyte AI, argon inlet V, valve for the separation between the electrochemical pre-chamher and the main UHV chamber SM, sample manipulator SP, sorption pump P, the turbomolecular pumps M, mass spectrometer S, sputter gun SC, sample of single crystals R, x-ray emission tube L, low-energy electron diffraction system H, heat lamp X, x-ray photoelectronic spectrometer and T, transfer rod with sample holder. 

A typical construction of a pH glass electrode is shown in Figure 13.6. For measurement, only the bulb need be submerged. There is an internal reference electrode and electrolyte (Ag AgCl Cl ) for making electrical contact with the glass membrane its potential is necessarily constant and is set by the concentration of HCl. A complete cell, then, can be represented by... 

The reference electrode for EP experiments is usually prepared by deposition of a continuous porous gold film onto the solid electrolyte. In the case of YSZ (an 0 -conducting solid electrolyte), the potential of this reference electrode is determined by the redox reaction... 

The normal hydrogen electrode (NHE) is the primary reference electrode and is used to define the accepted scale of standard potentials in aqueous media. It is also one of the most reproducible electrodes that are available. The hydrogen electrode has been successfully employed in dipolar aprotic solvents however, it is not frequently used. The aqueous saturated calomel electrode (SCE), connected to the electrolyte under study by a non-aqueous salt bridge, has become the reference electrode of choice for most investigators. Whether it is the SCE that is used, or any other suitable reference electrode for a given solvent, junction potentials will exist between the reference electrode and the electrolyte under study. These junction potentials will affect electrode potential measurements and will vary from one solvent/electrolyte system to another. In addition, the instability of the SCE in non-aqueous solvents has been noted. ... 

This difference between the Galvani potential differences of two volume phases is the so-called Donnan potential (Donnan in 1911). For more detailed information and special cases of membrane potentials, cf. Koryta [5]. If such a semipermeable membrane is permeable only for one ionic species, the Donnan potential can be used for the determination of the activity of this ionic species in an analyte solution. This is what ion-selective membrane electrodes are used for. Figure II.9.4 depicts schematically such an electrode [6]. The membrane separates two aqueous electrolyte solutions. Solution I is the so-called inner solution with a well-defined activity of the analyte Solution II is the so-called outer solution with the unknown activity of the analyte a. Two equal reference electrodes, for instance, silver/silver chloride electrodes, are immersed in solutions I and II and are connected via a voltmeter with high input impedance. Then for the overall cell potential it follows that... 

Ostwald antedated Nernst by several years in his proposal of a reference electrode for the electromotive series. He was, of course, aware of the early attempts to prepare reference electrodes. He specifically mentions that in earlier times amalgamated zinc in concentrated zinc sulfate solution had been used as a reference electrode, that this combination when used as a reference electrode Q) gave liquid junction potentials with other electrolytes which could not be calculated, and that the electrode also contaminated the experiments. 

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