Standard Hydrogen Electrode scale

Under open-circuit conditions the catalyst potential UwR=Urhe takes values of the order 0.4-0.85 V, that is -0.35 to +0.1 V on the standard hydrogen electrode scale (she), depending on the hydrogen to oxygen ratio. 

All values taken from literature sources are given on the standard hydrogen electrode scale. If necessary, conversions were based on standard values for the involved reference electrodes. If a single report contains several values measured with different concentrations of the electrolyte under investigation, all values are included in the table in order to allow insight into the influence of the concentration. 

Thus the proportionality of differences is preserved as we change from a standard hydrogen electrode scale in volts to a silver-silver chloride scale in millivolts or vice versa. 

Fig. 6.12, in which potential Ex has been measured against a secondary reference electrode and it is necessary to convert it to the standard hydrogen electrode scale which defines the zero . Thus, if the potential measured against Ag/AgCl (sat.) KC1//... is —99 mV, it is —142 mV versus SCE, —451 mV versus Fc/Fc+ redox electrode, but +100 mV versus SF1E. 

Figure 8.18. Measured redox potentials in a deep groundwater. Experimental values of the measured redox potentials (recalculated to the standard hydrogen electrode scale) versus (3pH + log[Fe ]). The concentration of [Fe J has been obtained from the analytical determinations by correction for the complex formation with carbonate. The notation refers to the different test sites. The full-drawn line has been calculated using the selected value of the standard potential E. The straight line has the theoretical Nemstian slope of +0.056 V, at the temperature of measurements. (Adapted from Grenthe et al., 1992.)...

Plots of = f(log [SeOj ])pn and E = f(pH)se(iv) at 293 K were linear and had the slopes 14.5 and 87.5 mV, respectively. These values are close to the Nemstian slopes required by Reaction (A.36) and the authors concluded that the half-cell was reversible. The potential, recalculated to the standard hydrogen electrode scale, measured at [SeOj ] = 1 M and [OH ] = 1 M was - 0.364 V and assigned as the standard potential of the redox-couple in alkaline solution. The authors noted this value Is close to the value in [56LAT], but they did obviously not know that this datum is in error by about 0.03V. 

It is noted in Fig. 6.1 that the experimentally measured potential, as measured against any given reference electrode (e.g., the saturated calomel electrode, SCE), is denoted as Eexp meas ref. When converting this potential to the standard hydrogen electrode scale (SHE), the following relationship applies ... 

Figure 16.6 Metallocene redox potential on the standard hydrogen electrode scale.

It is of great theoretical interest to relate a Galvani potential difference to the absolute potential of a clearly defined species, e.g., the potential of a free electron in vacuum. This defines an absolute potential scale (see Chap. 15), for which the precise relation to the conventional standard hydrogen electrode scale for aqueous solutions is still debated. Hence, a consistent system based on the standard hydrogen electrode (SHE) definition still serves the fundamental needs of measuring and calculating redox equilibria, and coupled chemical equihbria in aqueous systems. The relation of the electrode potentials in nmiaqueous systems, be they Uquid or solid, to the (aqueous) SHE, is of fundamental importance in chemistiy because nonaqueous systems play an important role in modem technologies and research, and in many cases aqueous and nonaqueous systems are even directly coupled, as, e.g., in ion partition systems. 

Laboratory experiments have shown that IGSCC can be mitigated if the electrochemical potential (ECP) could be decreased to —0.230 V on the standard hydrogen electrode (SHE) scale in water with a conductivity of 0.3 ]lS/cm (22). This has also been demonstrated in operating plants. Equipment has been developed to monitor ECP in the recirculation line and in strategic places such as the core top and core bottom, in the reactor vessel during power operation. 

It may be noted that the standard hydrogen electrode is rather difficult to manipulate. In practice, electrode potentials on the hydrogen scale are usually... 

Eq. (8)] represents by definition the zero point of the electrochemical potential scale (standard hydrogen electrode, often denoted SHE). 

Figure 2 illustrates the resulting situation. Due to the strong acidic solution in the battery, it corresponds lo Fig. 1 for small pH values, but here the electrode potential is drawn on the vertical axis. The values are referred to the above-mentioned standard hydrogen electrode. To enlarge the scale, the range between 0 and 1.2 V is omitted. 

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. 

Experimental Values of the Potential of the Standard Hydrogen Electrode in the UHV Scale... 

Figure 7.12 shows the relationship between the standard oxygen electrode (soe) scale of solid state electrochemistry, the corresponding standard hydrogen electrode (she) scale of solid state electrochemistry, the standard hydrogen electrode (she) scale of aqueous electrochemistry, and the physical absolute electrode scale. The first two scales refer to a standard temperature of 673.15 K, the third to 298.15 K. In constructing Figure 7.12 we have used the she aqueous electrochemical scale as presented by Trasatti.14... 

On the scale of the standard hydrogen electrode (SHE), we have Ei = 0 for reaction... 

Thus, the temperature coefficient of Galvanic potential of an individual electrode can be neither measured nor calculated. Measured values of the temperature coefficients of electrode potentials depend on the reference electrode employed. For this reason a special scale is used for the temperature coefficients of electrode potential It is assumed as a convention that the temperature coefficient of potential of the standard hydrogen electrode is zero in other words, it is assumed that the value of Hj) is zero at all temperatures. By measuring the EMF under isothermal conditions we actually compare the temperature coefficient of potential of other electrodes with that of the standard hydrogen electrode. 

The expression for the potential of electrodes of the second kind on the hydrogen scale can be derived from the affinity of the reaction occurring in a cell with a standard hydrogen electrode. For example, for the silver chloride electrode with the half-cell reaction... 

This type of counter electrode is defined as a reference electrode. As we will see in Chapter 3, Section 1.2, at 25° C the saturated calomel electrode (SCE) has a potential of +0.2415 V with respect to the standard hydrogen electrode (NHE), which, although difficult to use, is the internationally accepted standard for the potential scale, having conventionally E° — 0.000 V. 

The relative electrode potential nhe referred to the normal (or standard) hydrogen electrode (NHE) is used in general as a conventional scale of the electrode potential in electrochemistry. Since the electrode potential of the normal hydrogen electrode is 4.5 or 4.44 V, we obtain the relationship between the relative electrode potentiEd, Ema, and the absolute electrode potential, E, as shown in Eqn. 4-36 ... 

While this potential cannot he determined for a single electrode, a potential can be derived if the potential of the other electrode in a cell is defined, i.e. the potential of the standard hydrogen electrode (SHE) is arbitrarily taken as 0.(XXX)V. In this way. a potential scale can then be devised for single electrode potentials - see Section 3.2. t The abbreviation emf , in upright script, is often used in other lextNmks as a direct , i.e. non-variable, acronym for the electromotive force. Note, however, that in this present text it is used to represent a variable (cell potential) and is therefore. shown in italic script. 

Having revised a few basic electrochemical ideas, such as the nature of reference electrodes, the standard hydrogen electrode and the scale based on it, we next looked briefly at thermodynamic parameters such as the electrode potential E, the standard electrode potential f and emf, and then discussed how AG, AH and AS (where the prime indicates a frustrated cell equilibrium ) may be determined. 

We have seen already that an absolute potential at an electrode cannot be known, so, in accord with all other electrochemistry, it is the potential difference between two electrodes which we measure. However, if the potential of the electrode of interest is cited with respect to that of a second electrode having a known, fixed potential, then we can know its voltage via the concept of the standard hydrogen electrode (SHE) scale (see Section 3.1). We see that a reliable value of overpotential requires a circuit containing a reference electrode. 

When two interval scales are used to measure the amount of change in the same property, the proportionality of differences is preserved from one scale to the other. For example. Table 1.4 shows reduction potentials of three electrochemical half-cell reactions measured in volts with reference to the standard hydrogen electrode (SHE, E°) and in millivolts with reference to the standard silver-silver chloride electrode (Ag/AgCl, ). For the SHE potentials the proportion of differences between the intervals +0.54 to +0.80 and +0.34 to +0.80 is... 

E is the standard equilibrium potential, i. e. the potential corresponding to unit activity and RTF. The dissolution reaction leads to the development of an electrical double layer at the iron-solution interface. The potential difference of the Fe/Fe " half cell cannot be measured directly, but if the iron electrode is coupled with a reference electrode (usually the standard hydrogen electrode, SHE), a relative potential difference, E, can be measured. This potential is termed the single potential of the Fe/Fe electrode on the scale of the standard hydrogen couple H2/H, the standard potential of which is taken as zero. The value of the equilibrium potential of an electrochemical cell depends upon the concentrations of the species involved. 

Chapter 2) apply. The standard reference half-cell is reaction 15.6, the standard hydrogen electrode (SHE), and the standard conditions are those listed in Section 2.3, although for our purposes the molar concentration scale (mol L 1) can generally be used without significant loss of precision. We will simplify matters further, for illustrative purposes, by equating activities with molar concentrations our numerical results will therefore be only approximate, except where concentrations are very low. A thermodynamically acceptable treatment would require the calculation or measurement of ionic activities or, at the very least, maintenance of constant ionic strength, as outlined in Section 2.2. 

In the last section it was shown that instead of representing an electrode potential on a relative scale (arbitrarily setting the standard hydrogen electrode potential equal to zero), it is possible to numerically calculate the actual value of the latter, with a reference state of zero energy for the stationary electron at infinity in a vacuum. 

In calculating this "absolute or "vacuum scale potential of the standard hydrogen electrode, the expression quoted as an electrode potential was... 

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