Reference electrode for electrolytic reduction

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... 

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"). 

Figure 11.8 Simplified schematic to illustrate possible sources of fluctuations in corrosion current, /(-orr or corrosion potential measured at a distant reference electrode, for general corrosion with a diffusion-limited cathodic reaction such as oxygen reduction. Fluctuations leading to fluctuations in can be in (1), the transport rate of the cathodic reagent, leading to changes in diffusion-limited current (2) and (3), the relative areas of the anodic and cathodic processes, caused for example by detachment of surface scales or by changes in the electrode kinetics of these processes caused for example by the addition of corrosion inhibitors or change in surface concentration of such inhibitors (4), in the solution resistance between cathodic and anodic areas, if these are spatially separated, caused for example by fluctuations in local electrolyte composition itself linked to the occurrence of the corrosion reaction.

At high oxygen pressures, oxide phases show defect electron (hole) conduction (oxidation semiconduction) and at low oxygen pressures excess electron conduction (reduction semiconduction). The transport number of excess electrons in Zro.ssCao.isOi.ss as a function of the oxygen partial pressure could be determined by measurements with a Ca,CaO/air cell [79]. The hole conduction of zirconia-based solid electrolytes was noticed for the first time when cells with Ni,NiO reference electrodes for gas potentiometry [44,91] were tested in air. The harmful oxygen permeability was measured potentiometrically in 1965 [92]. 

Several significant electrode potentials of interest in aqueous batteries are listed in Table 2 these include the oxidation of carbon, and oxygen evolution/reduction reactions in acid and alkaline electrolytes. For example, for the oxidation of carbon in alkaline electrolyte, E° at 25 °C is -0.780 V vs. SHE or -0.682 V (vs. Hg/HgO reference electrode) in 0.1 molL IC0 2 at pH [14]. Based on the standard potentials for carbon in aqueous electrolytes, it is thermodynamically stable in water and other aqueous solutions at a pH less than about 13, provided no oxidizing agents are present. 

The following explanation can be provided. With Cu2+ ions there is a tendency for them to be reduced to Cu metal and precipitated on the electrode, which is reflected by a positive standard reduction potential (+ 0.34 V). For Zn metal there is a tendency for it to be oxidized to Zn2+ ions and dissolved in the electrolyte, which is reflected by a negative standard reduction potential (- 0.76 V). In fact, with Zn one could speak of a positive oxidation potential for the electrolyte versus the electrode, as was often done formerly however, some time ago it was agreed internationally that hence forward the potentials must be given for the electrode versus the electrolyte therefore, today lists of electrode potentials in handbooks etc. always refer to the standard reduction potentials (see Appendix) moreover, these now have a direct relationship with the conventional current flow directions. 

Fig. 28 Reductive electrochemistry data for (72). Cyclic voltammetric curves for a 0.1-mM CH2CI2 solution of (72) at 100 mV s , glassy carbon as a working electrode, Pt-mesh as a counter electrode, and a Ag wire as a quasi-reference electrode, T = 25 °C, TBAPFs (0.1 M) was used as supporting electrolyte.

For electrochemical work it is important to know the limiting potentials that may be applied in oxidative, anodic, or reductive, cathodic, scans of solutions in which solutes can undergo redox reactions without the solvent being oxidized or reduced. These limits constitute the electrochemical window for the solvent. However, the breadth of this window, in terms of the applicable voltages, depends not only on the solvent itself, but also on the material of the working electrode involved, the reference electrode against which the potentials are measured, and the nature of the supporting electrolyte present. 

The majority of controlled-potential electrochemistry has been carried out at mercury-pool electrodes. This is because of the vast amount of reference data available from polarography. Furthermore, the uniform and reproducible surface, and the high voltage for solvent reduction make the mercury pool particularly attractive relative to solid electrodes. As with electrodeposition, controlled-potential electrolysis rates are dependent on electrode area, stirring rates, solution volume, solution temperature, and supporting electrolyte. If the diffusion layer is uniform and the applied potential is such that one is on the diffusion plateau, the electrolysis obeys the relation... 

Figure 6 presents a scheme of an electrolysis cell for the isolation of reduction and oxidation products of nonaqueous solutions [15]. The electrolyte of the W.E. solution must be an alkyl ammonium salt because the reduction products of most of the commonly used solvents in the presence of metal cations precipitate as insoluble metal salts. The counter- and reference electrode compartments are separated from the working electrode compartment by two frits each. The separating units have pipes which enable the sampling of their solutions in order... 

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