Potential electrode conversion scale

Fig. 2.1 The electrochemical potential scale and electrode potential conversion at 25°C.

Electrode potentials are customarily tabulated on the standard hydrogen electrode (SHE) scale (although the SHE is never actually used experimentally because it is inconvenient in many respects). Therefore, conversion of potentials into the UHV scale requires the determination of E°(H+/H2) vs. UHV. According to the concepts developed above, such a potential would measure the energy of electrons in the Pt wire of the hydrogen electrode, modified by the contact with the solution. 

Figure 5 Number line for conversion of electrode potentials among different reference electrode scales.

Finally, electrode potential and acidity data were used by Ng et al. to study ligand effects on CpM(PR3)2H2 and TpM(PPh3)2(// -H2)+ (M = Ru, Os) complexes under similar conditions [31] these data are included in Table 3. The Ru-H BDE determined by Ng for CpRu(PPh3)2H2+ (Table 3, entry 19 289 kJ mol" ) is in near perfect agreement with that of Angelici (Table 2, entry 25 286 kJ mol ) but somewhat different from the value derived from Morris data (Table 3, entry 1 303 kJ moF ). The source of the discrepancy might partly be the assumptions and conversions made to align the pXa and electrochemical reference scales of the different solvents used. 

A survey of the literature, old and recent, reveals that electrochemical experiments are conducted under widely different conditions. Reported electrode potentials for one given compound can vary significantly even under apparently quite similar conditions when published values from different sources are compared. Frequently, this can be caused by erroneous reporting of data or improper conversion of electrode potentials from the scale of one reference electrode to another. Furthermore, the use of different solvents, supporting electrolytes, and physical reference electrodes adds other, physically well understood, reasons why E° data differ, and in many instances should actually differ considerably. It is important at least to understand these effects, and if possible to make reasonable corrections for them. Obviously, there is a definite need to standardize as much as possible the way in which electrode potential data are reported. Errors or discrepancies in E° values will easily lead to significant errors in derived bond energy data. 

H2 dissociation, H solvation, conversion of electrode potentials to the Fc scale, and entropy contributions that convert the free-energy based quantities (BDEq) obtained from E° and pK data to enthalpy-based BDEs. These terms have been extensively discussed and evaluated." Equation (7), Scheme 3 is the simple equation that is used... 

In this chapter we introduce and discuss a number of concepts that are commonly used in the electrochemical literature and in the remainder of this book. In particular we will illuminate the relation of electrochemical concepts to those used in related disciplines. Electrochemistry has much in common with surface science, which is the study of solid surfaces in contact with a gas phase or, more commonly, with ultra-high vacuum (uhv). A number of surface science techniques has been applied to electrochemical interfaces with great success. Conversely, surface scientists have become attracted to electrochemistry because the electrode charge (or equivalently the potential) is a useful variable which cannot be well controlled for surfaces in uhv. This has led to a laudable attempt to use similar terminologies for these two related sciences, and to introduce the concepts of the absolute scale of electrochemical potentials and the Fermi level of a redox reaction into electrochemistry. Unfortunately, there is some confusion of these terms in the literature, even though they are quite simple. 

Fig. 6.12 Conversion of an unknown ( ) potential measured with one reference electrode to another reference electrode scale (all values are in millivolts)...

Salt bridges (particularly those that employ saturated KC1) also have been widely used in the conversion of polarographic and other potentials from one reference-electrode scale to another. This conversion is commonly made by direct comparison of the two reference (ref) electrodes in the cell... 

In this laboratory, you will construct a number line for reference electrode conversions, measure the corrosion potentials of several alloys in a salt water solution, construct a galvanic series with two different reference electrodes, and convert the two galvanic series to the NHE scale to determine if they agree (as they should). This lab will demonstrate some of the concepts discussed in Chapter 2. 

Sugars with a potential aldehyde function can be reduced electrochemically at electrodes of mercury or amalgamated lead [23]. The rate of this process is controlled by the rate for the conversion of the cyclic to the open-chain form of the sugar. A technical-scale plant [24] for the conversion of glucose to either sorbitol or mannitol was operated in the past, but this method has largely been ousted by other processes. Glucose is converted to... 

On the laboratory scale, it may be helpful to use three-electrode cells, where the electrochemical potential of the working electrode is set against that of a reference electrode, itself connected to a potentiostat. The purpose of a potentiostat is to fix precisely the reduction (or oxidation) potential necessary for the complete conversion of an organic substrate. At the end of the reaction, the electrochemical current has totally vanished and the concomitant use... 

Figure 5.21a presents, on a logarithmic scale, the anodic CTs calculated on a theoretical basis, with and without considering the interaction between lithium ions in the Lii 8Mn2O4 electrode under the cell-impedance-controlled constraint with the conversion factor/= 0.2 at the potential step across the disorder-order and backward transition points. In the case when no interaction is assumed, the theoretical CT does not display any transition time, but rather shows a monotonic increase of its slope from an almost flat value to one of infinity. 

The basic structure of the battery is the same as the electrodialyzer a plurality of a pair of cation and anion exchange membranes is alternately installed to form the concentrated and dilute compartment between electrodes at both ends. Then the concentrated and dilute solutions flow into each compartment and electric power based on the membrane potential is taken out from the electrodes. Various ion exchange membranes have been examined to calculate the energy conversion efficiency.284 A maximum power would be 0.33 Wm-2/pair when 0.57moll-1 solution (concentrated stream) and 0.026 mol l-1 solution (the dilute stream) are fed into a electrodialyzer with 30 pairs of cation and anion exchange membrane (effective membrane area 232 cm2).283 Also, it is calculated to be 0.6 W m-2/pair of electric power in an ideal scale-up based on experimental data when 30 gl-1 and 3 gl-1 solutions flow into the concentrated and dilute compartments.285... 

An electrode is inexpensive when compared with most chemical reagents. It is immobile, and thus causes less environmental and solubility problems than most chemical oxidants and reductants. It can change the polarity of reagents by oxidation or reduction ( Redox-Umpolung ) and in this way can shorten synthetic sequences. Controlled potential electrolysis allows the selective conversion of one out of several electrophores in a molecule. A technical scale-up causes in most cases lesser problems than the scale-up of a chemical reaction. These advantages and the wide choice of conversions have made electrolysis today at least for those that take the small effort to assemble an electrolysis cell and connect it to a d.c. power supply - to an attractive alternative and supplement for chemical synthetic methods. 

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