Electrochemistry junction potential

Both the - standard hydrogen electrode (SHE), which is the primary standard in electrochemistry [iv,v] and the relative hydrogen electrode (RHE) are widely used in aqueous acidic solutions. In RHE the nature and concentration of acid is the same in the reference and the main compartments. In general, it is advantageous to use the same solution in both compartments to decrease the - junction potential. By the help of RHE the -> activity effect can also be eliminated when the -> pH dependence of a — redox reaction is to be determined, since the H+ ion activity influences both the redox reactions under study and the redox reaction occurring in the reference system (1/2H2 -> H+ + e-) in the same way. 

The general subject of liquid junction potentials is important in electrochemistry. These junctions often involve different electrolytes on either side of the boundary in which case the potential difference is more difficult to estimate as described later in this chapter. The experimental cells considered in this section demonstrate that only those systems which do not have a liquid junction can be treated exactly by thermodynamics. However, most cells used in electrochemistry have a liquid junction, and therefore can only be treated approximately. This is especially true for the cells used to perform electroanalysis. 

Internal standards are routinely used in analytical measurements to aid in the quantification of analyte signals in chromatography, or as reference energy standards in NMR. An internal standard can be used in electrochemistry as a reference potential standard in circumstances that an RE alone does not provide an adequately known reference potential in a particular system. This may be the case if a QRE (Section 4.3.4) is used or if large junction potentials are encountered within an electrochemical cell, as with an aqueous RE in a... 

The most common reference electrode systems used in aqueous solutions are Ag/AgCl and the calomel electrode. If aqueous-based references are used in nonaqueous solution, however, large liquid junction is produced and often more serious, aqueous contamination of the nonaqueous cell occurs. Thus this combination is not recommended. The use of an Ag/Ag non-aqueous-based reference is suggested for nonaqueous electrochemistry. To avoid large junction potentials, the RE solvent should be as close in nature as possible to the cell solvent system. Often potentials are calibrated with a standard, such as ferrocene or cobaltocene. Suggested standards are listed in Table 2-2, along with reduction potentials and other properties. Construction of an Ag/Ag reference for nonaqueous use is shown in Figure 2-6. Reference electrodes can drift with time and must be carefully maintained. 

As electrochemistry moved into mixed and nonaqueous electrolytes it became of interest to compare potentials in different media. Serious problems preventing comparison are the Uquid junctimi potentials between different electrolytes. Such liquid junction potentials also occur in the measurement in aqueous systems, but they are generally suppressed by a salt bridge. Salt bridges for aqueous systems usually consist of (saturated) solutions of KCl or NH4NO3. For both KCl and NH4NO3 similar mobilities for the cation and the anion of the respective salt were measured in aqueous solutions. Thus the liquid junction potential between two aqueous electrolytes cmmected via such a bridge should be smaller than the experimental error (see Chap. 1). Data in aqueous systems without liquid junction potentials are obtained from measurements in cells without transference such as ... 

With other ion-selective electrodes, the liquid junction potential can be estimated and corrected with the equations given below, the calibration can be performed in a similar electrolyte background, or the electrolyte background can be adjusted by adding a buffer electrolyte (TISAB). In some cases, a liquid junction is not necessary, such as with potentiometric gas sensing probes and in some dynamic electrochemistry approaches. 

If the tunnel junction of Fig. 1 a is simply immersed in an electrolyte, the polarization between the tip and the sample will promote an electrolysis. A bi-potentiostat is necessary to ensure real tunneling between the sample and the tip. Such a device, classically used in electrochemistry, enables to split the tunnel junction into two sol-id/liquid interfaces, independently polarized against a reference of potential (Fig. 1 b). Using this configuration, also referred to as the four-electrode configuration and introduced very early by several groups, it is possible to avoid any electrochemical transfer between the sample and the tip [25,26]. The reference potential is an electrode whose potential is well defined and constant with respect to the vacuum level. The sample is biased against the reference electrode to monitor reactions at the surface, just as in a classical electrochemical cell. The tip potential is adjusted... 

Traditionally, electrochemical equilibria are explained in terms of thermodynamic cell potentials. However, in electro analytical applications, such a description is of little use, because one almost always uses a non-thermodynamic measurement, with a reference electrode that includes a liquid junction. It is then more useful to go back to the basic physics of electrochemistry, i.e., to the individual interfacial potential differences that make up the total cell potential. This is the approach we will use here. 

The main objective of this chapter is to illustrate how fundamental aspects behind catalytic two-phase processes can be studied at polarizable interfaces between two immiscible electrolyte solutions (ITIES). The impact of electrochemistry at the ITIES is twofold first, electrochemical control over the Galvani potential difference allows fine-tuning of the organization and reactivity of catalysts and substrates at the liquid liquid junction. Second, electrochemical, spectroscopic, and photoelectrochemical techniques provide fundamental insights into the mechanistic aspects of catalytic and photocatalytic processes in liquid liquid systems. We shall describe some fundamental concepts in connection with charge transfer at polarizable ITIES and their relevance to two-phase catalysis. In subsequent sections, we shall review catalytic processes involving phase transfer catalysts, redox mediators, redox-active dyes, and nanoparticles from the optic provided by electrochemical and spectroscopic techniques. This chapter also features a brief overview of the properties of nanoparticles and microheterogeneous systems and their impact in the fields of catalysis and photocatalysis. 

Electrochemistry at liquid/liquid interfaces has progressed markedly in the past 30 years. Excellent work on modified liquid/liquid interface with lipids and nanoparticles have been reported (75, 78, 124-127). Droplet electrodes and three-phase jnnctions have made this field more popular and versatile (102, 103, 128). The ET induced IT reactions at three-phase junction have been employed to obtain the log P of different drugs at W/n-octanol interfaces (102, 129, 130). New and less toxic solvents, such as room tanperature ionic liquids (RTILs) have replaced organic solvents to form W/RTIL interfaces (131, 132). However, from a theoretical point of view, the key aspects of potential distribution remain the major challenge. Only a few biological applications have been so far reported based on the techniques developed from this field. 

When a metal is placed in contact with an electrolyte, a potential difference is observed at the liquid-metal interface, as noted in Chapter 2. This is similar to the work-function potential difference which occurs when two dissimilar metals are brought into contact, or the potential difference associated with a semiconductor p n junction. The value of potential difference associated with a metal electrode-electrolyte interface is a function of the metal and contacting electrolyte. Theoretical treatment of this situation is complex and one should refer to a text on electrochemistry such as those by Macinnes (1961) or Newman (1973). Certain types of electrodes are extremely sensitive to various trace impurities in the contacting electrolyte and may react quite differently in seemingly similar circumstances. 

For nonaqueous electrochemistry, lUPAC recommended the use of a redox couple such as ferrocene/ferrocenium ion (Fc/Fc" ) as an internal standard [26]. An alternative to the liquid junction electrode is one based on an entirely solid-state design. Peerce and Bard [27] fabricated such an electrode by coating poly (vinylferrocene) (PVFc) on platinum. The polymer-coated electrode was brought to a 1 1 ratio of ferrocene to ferrocenium by poising the electrode at the PVTc/Fc" half-wave potential (0.39 V vs. SCE). Although this electrode maintained a constant, reproducible potential in deaerated acetonitrile over 21 h, it was unstable in other... 

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