Standard Reference Electrodes

Figure C2.8.3. A tliree-electrode electrochemical set-up used for the measurement of polarization curves. A potentiostat is used to control the potential between the working electrode and a standard reference electrode. The current is measured and adjusted between an inert counter-electrode (typically Pt) and the working electrode.

Ideally, one would prefer to compare anodic and cathodic potential limits instead of the overall ionic liquid electrochemical window, because difference sets of anodic and cathodic limits can give rise to the same value of electrochemical window (see Figure 3.6-1). However, the lack of a standard reference electrode system within and between ionic liquid systems precludes this possibility. Gonsequently, significant care must be taken when evaluating the impact of changes in the cation or anion on the overall ionic liquid electrochemical window. 

The thermodynamic driving force behind the corrosion process can be related to the corrosion potential adopted by the metal while it is corroding. The corrosion potential is measured against a standard reference electrode. For seawater, the corrosion potentials of a number of constructional materials are shown in Table 53.1. The listing ranks metals in their thermodynamic ability to corrode. Corrosion rates are governed by additional factors as described above. 

One distingnishes practical and standard reference electrodes. A standard RE is an electrode system of particnlar confignration, the potential of which, nnder specified conditions, is conventionally taken as zero in tfie corresponding scale of potentials (i.e., as the point of reference nsed in finding tfie potentials of otfier electrodes). Practical REs are electrode systems having a snfficiently stable and reproducible value of potential which are nsed in the laboratory to measure the potentials of other electrodes. The potential of a practical reference electrode may difier from the conventional zero potential of the standard electrode, in which case the potential of the test electrode is converted to this scale by calculation. 

The quantity kconv = exp (anFE0,/RT) is the value of the rate constant of the electrode reaction at the potential of the standard reference electrode and will be termed the conventional rate constant of the electrode reaction. It can be found, for example, by extrapolation of the dependence (5.2.36) to E = 0, as... 

If the concentration of the electrolyte is sufficient then, as we saw above, all the potential drop at the interface will take place between the metal and the OHP, and the value of — A< 0) ca i be replaced by E — fc °, where E, E° are potentials of the electrode with respect to a standard reference electrode. We note that E° is the potential at which no current flows, provided [O]0 = [R]o n equation (1.26). We then obtain ... 

It is impossible to measure the potential of a half-cell directly and a reference half-cell must be used to complete the circuit. The hydrogen electrode (Figure 4.3) is the standard reference electrode against which all other halfcells are measured and is arbitrarily attributed a standard electrode potential of zero at pH 0. Because it is difficult to prepare and inconvenient to use, the... 

In this present book, we will look at the analytical use of two fundamentally different types of electrochemical technique, namely potentiometry and amper-ometry. The distinctions between the two are outlined in some detail in Chapter 2. For now, we will anticipate and say that a potentiometric technique determines the potential of electrochemical cells - usually at zero current. The potential of the electrode of interest responds (with respect to a standard reference electrode) to changes in the concentration of the species under study. The most common potentiometric methods used by the analyst employ voltmeters, potentiometers or pH meters. Such measurements are generally relatively cheap to perform, but can be slow and tedious unless automated. 

The voltage we measure is characteristic of the metals we use. As an additional example, unit activity solutions of CuCE and AgCl with copper and silver electrodes, respectively, give a potential difference of about 0.45 V. We could continue with this type of measurement for aU the different anode-cathode combinations, but the number of galvanic cells needed would be very large. Fortunately, the half-reactions for most metals have been calculated relative to a standard reference electrode, which is arbitrarily selected as the reduction of hydrogen ... 

It has been stressed that the reaction order must be measured at a constant potential difference across the electrode/solution interface at which the reaction occurs. It will be recalled that the condition At > = constant is tantamount to the condition that the potential of the electrode relative to a standard reference electrode is constant (Section 7.5.7.3). Thus, in order to obtain in practice the reaction order of, say, species A, one would measure current densities obtained at a certain potential E referred to a standard electrode potential, in solutions containing various concentrations of A and constant concentrations of all other reactants (Fig. 7.73). The potential E must be chosen sufficiently far from the reversible potential E, so that the exponential law (Section 7.2.3b.2) applies even at the highest concentrations of the given species in deelectronation reactions and at the lowest concentrations in electronation reactions. 

The quantity d< ) was discussed earlier (see Section 6.3.10). It is the Galvani potential difference and refers to the potential difference between the interior of the metal and the interior of the solution adjacent to the metal. The actual electrode potential as measured against a standard reference electrode is given by... 

The most common electrode of this type is the saturated calomel electrode (SCE) which consists of mercury in contact with a layer of insoluble Hg2Cl2 immersed in a saturated aqueous solution of KC1. The SCE is used as a secondary standard reference electrode. At 25°C it has an electrode potential of + 0.2415 V. 

In constructing an electric cell for potentiometric titrations it is necessary, of course, to use a second electrode to complete the circuit, in addition to the measuring electrodes (commonly called indicator electrodes ) described above. Ideally the second electrode would be a hydrogen electrode which (as explained in the entry on electrode potential) is the standard reference electrode for which die potential, in equilibrium with its ions, is defined as zero. Since it is awkward to use, other electrodes of known potential, such as the calomel electrode or the glass electrode, are commonly used as reference electrodes. The arrangement of the apparatus is as shown in Fig. 1. 

With this choice of standard reference electrode, the entire potential of the cell... 

The following table lists some standard electrode potentials (in V) in various solvents. The rubidium ion, which possesses a large radius and shows a low deformability, has a rather low and constant solvation energy in all solvents.1 As a result, the rubidium electrode is taken as a standard reference electrode in all solvents. 

Any surface (typically a piece of metal) on which an electrochemical reaction takes place will produce an electrochemical potential when in contact with an electrolyte (typically water containing dissolved ions). The unit of the electrochemical potential is volt (TV = 1JC1 s 1 in SI units).The metal, or strictly speaking the metal-electrolyte interface, is called an electrode and the electrochemical reaction taking place is called the electrode reaction. The electrochemical potential of a metal in a solution, or the electrode potential, cannot be determined absolutely. It is referred to as a potential relative to a fixed and known electrode potential set up by a reference electrode in the same electrolyte. In other words, an electrode potential is the potential of an electrode measured against a reference electrode. The standard hydrogen electrode (SHE) is universally adopted as the primary standard reference electrode with which all other electrodes are compared. By definition, the SHE potential is OV, i.e. the zero-point on the electrochemical potential scale. Electrode potentials may be more positive or more negative than the SHE. 

Here Eq is the equilibrium electrode potential difference between the two electrodes of the galvanic cell. It is now possible to choose the standard reference electrode as the hydrogen electrode, for which the chemical potentials jXj of the constituents and H2 are taken to be zero at standard temperature and pressure. The equilibrium condition 7.14 then becomes... 

As for semiconductor/metal contacts, a change in the Fermi level of the liquid phase should result in a different amount of charge transferred across the semicondnctor/liqnid junction. For semiconductor/liquid junctions, the important energetic trends for a series of different liqnid contacts can thns be determined by measuring the solntion redox potential relative to a standard reference electrode system. Within this model, solutions with more positive redox potentials shonld indnce greater charge transfer in contact with n-type semicondnctors. 

This is a thermodynamically reversible process that often serves as a standard reference electrode, known as the reversible hydrogen electrode (RHE), for all other electrochemical processes. 

Having located the semiconductor band-edge positions (relative to either the vacuum reference or a standard reference electrode), we can also place the Fermi level of the redox system, E f, redox, on the same diagram. Energy diagrams such as those... 

In a redox electrode both components of the redox system are in solution, and their relative concentrations determine the potential of an inert electrode immersed in the system. Such electrodes have been used as reference electrodes only in special cases, because the potential is dependent on two concentrations, the ratio of which must be kept constant. The primary standard reference electrode, the hydrogen electrode, has been used as practical reference electrode in certain case, as in HF [217,218]. 

Uief open-circuit potential of the reaction at the reference concentrations relative to a standard reference electrode of a given kind, V... 

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