Reference potentials

Alexander M H and Manolopoulos D E 1987 A stable linear reference potential algorithm for solution of the quantum close-coupled equations in molecular scattering theory J. Chem. Phys. 86 2044-50... 

Potentiometric measurements are made using a potentiometer to determine the difference in potential between a working or, indicator, electrode and a counter electrode (see Figure 11.2). Since no significant current flows in potentiometry, the role of the counter electrode is reduced to that of supplying a reference potential thus, the counter electrode is usually called the reference electrode. In this section we introduce the conventions used in describing potentiometric electrochemical cells and the relationship between the measured potential and concentration. 

Potentiometric electrochemical cells are constructed such that one of the half-cells provides a known reference potential, and the potential of the other half-cell indicates the analyte s concentration. By convention, the reference electrode is taken to be the anode thus, the shorthand notation for a potentiometric electrochemical cell is... 

The reference potential for other solutions containing Cf ions must be determined. Activated with Hg. 

The reference potential of the Ag-AgCl electrode in brackish water must be eorreeted for ehloride ion eontent (i.e., a ehange in ehloride ion eoneentration by a faetor of 10 shifts the referenee potential by about 50 mV in the positive direetion... 

Then, given an arbitrary potential, we compare it to the reference potential (A.5) adjusting the width of the latter Xp such that the instanton periods p are equal when the energies E are the same. In other words we require... 

The electrolysis is carried out at a reference potential of -2.4 volts vs a standard calomel electrode. An initial current density of 0.0403 amp/cm is obtained which drops to 0.0195 amp/cm at the end of the reduction, which is carried on over a period of 1,682 minutes at 15° to 20°C. The catholyte is filtered, the solid material is washed with water and dried. 430 g of the 2,3-bis-(3-pyridyl)-butane-2,3-diol is recrystallized from water, MP 244° to 245°C. 

However, in the case of stress-corrosion cracking of mild steel in some solutions, the potential band within which cracking occurs can be very narrow and an accurately known reference potential is required. A reference half cell of the calomel or mercury/mercurous sulphate type is therefore used with a liquid/liquid junction to separate the half-cell support electrolyte from the process fluid. The connections from the plant equipment and reference electrode are made to an impedance converter which ensures that only tiny currents flow in the circuit, thus causing the minimum polarisation of the reference electrode. The signal is then amplifled and displayed on a digital voltmeter or recorder. 

Finally, calomel electrodes (and more especially hydrogen electrodes) are not suitable for field measurements because they are not sufficiently robust. The calomel electrodes are however essential for calibrating the field reference electrodes. Saturated KCI calomel electrodes are the most suitable because there is then no doubt about the reference potential of the calibrating electrode. Lack of adequate calibration is a common cause of cathodic protection system mismanagement. 

FIGURE 6.4. Examining the strain hypothesis by a free-energy perturbation study. The results are plotted as a function of rather than A, for simplicity. The AG is given relative to the reference potential [-Kcos( Ci 0s)]. The figure demonstrates that the strain effect is not significant. 

In addition to defined standard conditions and a reference potential, tabulated half-reactions have a defined reference direction. As the double arrow in the previous equation indicates, E ° values for half-reactions refer to electrode equilibria. Just as the value of an equilibrium constant depends on the direction in which the equilibrium reaction is written, the values of S ° depend on whether electrons are reactants or products. For half-reactions, the conventional reference direction is reduction, with electrons always appearing as reactants. Thus, each tabulated E ° value for a half-reaction is a standard reduction potential. 

The potential that develops in an electrochemical system such as a fuel cell can also act to significantly influence the energies, kinetics, pathways, and reaction mechanisms. The double-reference potential DFT method [Cao et al., 2005] described earlier was used to follow the influence of an external surface potential on the reaction... 

The second term on the right-hand side of (5.1) is the energy required to transfer the charge qt associated with each particle of the ith species from a reference potential < ref to the electrostatic potential of the bulk electrolyte (solution), energy zero for the electrostatic potential. Further discussion on the importance of this aspect will be provided later. 

Reference electrodes. There are two types of reference electrodes (see the scheme in Section 1.3.1) (a) those constructed as a reference type and (b) those used as a reference type both types fulfil the requirement of a constant reference potential by either being non-polarizable or becoming non-polarized during the measurememt. 

Fig. 5.35 SNIFTIRS spectrum from a polished Pt electrode in 0.5 m LiC104 in propylene carbonate. Reference potential 2.00 V versus Li/Li+ electrode working potential 3.20 V versus Li/Li+ electrode. According to P. Novak et al.

This potential is related to the electrode potential, E (apart from an additive constant depending on the definition of the reference potential). The first term in Eq. (4.5.6), Af(j> characterizes the band bending... 

In the second case the potential can be modulated between two values (a reference and a sample potential) while the spectral frequencies are slowly scanned, or else the spectral data can first be collected at a reference potential, after which this is stepped to the sampling value where a second spectrum is obtained. The change in reflectivity AR/R is then computed,... 

Few (4 to 8) [26] spectrometer scans are taken, averaged and stored while the electrode potential is held at a reference potential and then at a working potential. In this way only a short time is passed between the transition from the reference to the working condition. The sequence is repeated many times in order to improve the signal-to-noise ratio. 

Most results of in situ IR studies on Pt in acidic methanol solutions so far have been obtained using a relatively fast (8.5-13.6) Hz) modulation of electrode potential. As already pointed out by Bockris [27], collection of spectral data alternatively at two potentials is not appropriate for processes which are not reversible to follow the change of potential. In this study the SPAIRS version of SNIFTIRS was performed by stepping the potential from a reference potential in the anodic direction, allowing sufficient time at each potential to reach stationary conditions. 

Substances formed at a given potential give rise to a diminution of the reflected intensity as compared with that at the reference potential. As a consequence, the relative reflection band presents a minimum ( negative going band ). On the contrary, the reflected intensity for substances consumed at a given potential present a maximum ( positive going band ). 

The electrolyte volume of the STM cells is usually very small (ofthe order of a 100 pi in the above-described case) and evaporation of the solution can create problems in long-term experiments. Miniature reference electrodes, mostly saturated calomel electrodes (SCE), have been described in the literature [25], although they are hardly used anymore in our laboratory for practical reasons Cleaning the glassware in caroic acid becomes cumbersome. For most studies, a simple Pt wire, immersed directly into solution, is a convenient, low-noise quasireference electrode. The Pt wire is readily cleaned by holding it into a Bunsen flame, and it provides a fairly constant reference potential of fcj>i — + 0.55 0.05 V versus SCE for 0.1 M sulfuric or perchloric acid solutions (+ 0.67 0.05 V for 0.1 M nitric acid), which has to be checked from time to time and for different solutions. 

It is also possible to build the order parameter directly into the potential energy function. This would enable one to determine the free energy difference between a given potential and a reference potential. In a sense, this is complementary to the previous discussion of reaction coordinates rather than further subdividing configuration space by various order parameters, we are considering multiple configuration... 

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