Electrochemistry reference electrode

Combination silver—silver salt electrodes have been used in electrochemistry. The potential of the common Ag/AgCl (saturated)—KCl (saturated) reference electrode is +0.199 V. Silver phosphate is suitable for the preparation of a reference electrode for the measurement of aqueous phosphate solutions (54). The silver—silver sulfate—sodium sulfate reference electrode has also been described (55). 

The solution to reference electrode instabiUty is the introduction of a third or auxiUary electrode. This particular electrode is intended to carry whatever current is required to keep the potential difference between the working and reference electrodes at a specified value, and virtually all potentiostats (instmments designed specifically for electrochemistry) have this three-electrode configuration. Its use is illustrated in Figure 3. 

The technique of AC Impedance Spectroscopy is one of the most commonly used techniques in electrochemistry, both aqueous and solid.49 A small amplitude AC voltage of frequency f is applied between the working and reference electrode, superimposed to the catalyst potential Uwr, and both the real (ZRe) and imaginary (Zim) part of the impedance Z (=dUwR/dI=ZRc+iZim)9 10 are obtained as a function of f (Bode plot, Fig. 5.29a). Upon crossplotting Z m vs ZRe, a Nyquist plot is obtained (Fig. 5.29b). One can also obtain Nyquist plots for various imposed Uwr values as shown in subsequent figures. 

The use of the soe scale is more convenient than the she scale in solid state electrochemistry since YSZ (an O2 conductor) is the most commonly used solid electrolyte and the metal/02 (po2 =1 atm)/YSZ (soe) electrode is the most commonly used reference electrode in solid state electrochemistry. 

What is the practical usefulness of the soe scale of solid state electrochemistry As in aqueous electrochemistry, it is limited but not trivial. When a potential Uwr of, e.g. -300 mV is measured in an YSZ solid state cell at 673 K vs a reference electrode at p02=l atm, the implication is that the work function of the reference electrode is 5.14 eV and that of the working electrode 4.84 eV regardless of the material of the two electrodes. 

The microarray electrodes used for solid state electrochemistry are a slight variation of the transistor decribed in Sect. 5.2.2 The most appealing feature is the location of all the necessary electrodes on a single microchip, the reference electrode being provided by the application of a droplet of silver epoxy to one of the gold micro electrodes (Fig. 7). 

Figure 2.1 (a) A schematic representation of the apparatus employed in an electrocapillarity experiment, (b) A schematic representation of the mercury /electrolyte interface in an electro-capillarity experiment. The height of the mercury column, of mass m and density p. is h, the radius of the capillary is r, and the contact angle between the mercury and the capillary wall is 0. (c) A simplified schematic representation of the potential distribution across the metal/ electrolyte interface and across the platinum/electrolyte interface of an NHE reference electrode, (d) A plot of the surface tension of a mercury drop electrode in contact with I M HCI as a function of potential. The surface charge density, pM, on the mercury at any potential can be obtained as the slope of the curve at that potential. After Modern Electrochemistry, J O M. 

The classic book on the topic remains Reference Electrodes by D. I. G. Janz and G. J. Ives, Academic Press, New York, 1961. This book is still worth consulting despite its age. One of the best articles is Reference electrodes for voltammetry by Adrian W. Bott, Current Separations, 1995, 14(2), 33. The article can be downloaded from http //www.currentseparations.com/issues/14-2/csl4-2d.pdf. Also, Electrochemistry by Carl H. Harnann, Andrew Hamnett and Wolf Vielstich, Wiley-VCH, Weinheim, 1998, has extensive discussions on reference electrodes. 

Figure 8.1 Mechanical electrochemistry measurement equipment 1-adjuster of rotation speed 2-model 273 3-plank 4-high speed motor 5-opposite electrode 6-reference electrode 7-digital pressure gauge 8-lift platform 9-medium 10-resin mattress 11-electrolytic cell 12-working electrode...

Other reference electrodes are discussed in standard electrochemistry textbooks (see the Bibliography). 

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. 

Fig. 28 Reductive electrochemistry data for (72). Cyclic voltammetric curves for a 0.1-mM CH2CI2 solution of (72) at 100 mV s , glassy carbon as a working electrode, Pt-mesh as a counter electrode, and a Ag wire as a quasi-reference electrode, T = 25 °C, TBAPFs (0.1 M) was used as supporting electrolyte.

As regards other coordination compounds of silver, electrochemical synthesis of metallic (e.g. Ag and Cu) complexes of bidentate thiolates containing nitrogen as an additional donor atom has been described by Garcia-Vasquez etal. [390]. Also Marquez and Anacona [391] have prepared and electrochemically studied sil-ver(I) complex of heptaaza quinquedentate macrocyclic ligand. It has been shown that the reversible one-electron oxidation wave at -1-0.75 V (versus Ag AgBF4) corresponds to the formation of a ligand-radical cation. Other applications of coordination silver compounds in electrochemistry include, for example, a reference electrode for aprotic media based on Ag(I) complex with cryptand 222, proposed by Lewandowski etal. [392]. Potential of this electrode was less sensitive to the impurities and the solvent than the conventional Ag/Ag+ electrode. 

Figure 8.9 Electrochemistry of functionalized nanotubes, 0.01 M tetrabutylammonium hexafhiorophosphate, THF solution 1.67 mg/mL. V—0.5 V/s. 7 — 25°C, and working electrode is Pt disk (r — 62.5 pm) potentials measured versus silver quasi-reference electrode (approximately —0.05 V versus SCE). Reproduced with permission from Ref. 120. Copyright 2004 American Chemical Society.

Scanning tunneling microscopy (STM), 787. 1157 bioelectrochemistry and, 1159 electrochemistry and. 1158 electrodeposition and. 1310 nanotechnology, 1345 piezoelectric crystal, 1158 tunneling current. 1157 underpotential deposition, 1313, 1315 Scavanger electrolysis, electrodeposition, 1343 Schlieren method, diffusion layer. 1235 Schmickler, 1495,1510 Schrodinger equation, 1456, 1490 Schultze 923,1497.1510 Screw dislocation, 1303, 1316, 1321, 1326 Secondary reference electrode, 815, 1109 Self-consumed electrode, 1040 Semiconductors... 

Electrochemistry is studied in cells, which consist of a working electrode (where the process to be studied occurs) and the counter-electrode necessary to complete the circuit in which one electrode emits electrons and the other receives them i.e., a direct current passes through the whole cell (There is a third electrode, the reference electrode, which is necessary to measure the overpotential of the working electrode see Section 6.3.4.)... 

Let us note one vital point, which is of methodological importance. It has been traditionally accepted in electrochemistry to choose the positive direction of the electrode potential

positive electrode charge. Here the zero potential is assumed to be that of the reference electrode, which coincides, within a constant, with the potential in the solution bulk (— oo). On the other hand, in physics of semiconductor surface the potential is usually reckoned from the value in the semiconductor bulk ( ) the enrichment of the surface with electrons, i.e., the formation of a negative space charge, corresponding to the positive potential of the surface. In particular, this statement directly follows from the Boltzmann distribution for electrons and holes in the space-charge region in a semiconductor ... 

The combined effects of electroneutrality and the Donnan equilibrium permits us to evaluate the distribution of simple ions across a semipermeable membrane. If electrodes reversible to either the M+ or the X ions were introduced to both sides of the membrane, there would be no potential difference between them the system is at equilibrium and the ion activity is the same in both compartments. However, if calomel reference electrodes are also introduced into each compartment in addition to the reversible electrodes, then a potential difference will be observed between the two reference electrodes. This potential, called the membrane potential, reflects the fact that the membrane must be polarized because of the macroions on one side. It might be noted that polarized membranes abound in living systems, but the polarization there is thought to be primarily due to differences in ionic mobilities for different solutes rather than the sort of mechanism that we have been discussing. We return to a more detailed discussion of the electrochemistry of colloidal systems in Chapter 11. 

Because there is no truly reliable reference electrode for use in non-aqueous solutions, various reference electrodes have so far been used. Thus, accurate mutual comparison of the potential data in non-aqueous systems is often difficult. In order to improve the situation, the IUPAC Commission on Electrochemistry proposed a method to be used for reporting electrode potentials [8]. It can be summarized as follows ... 

Clark oxygen electrode. [D. t. Sawyer, A. Sobkowiak, and J. L. Roberts, Jr., Electrochemistry for Chemists, 2nd eel. (New York Wiley. 1995).] A modern, commercial oxygen electrode is a three-electrode design with a Au cathode, a Ag anode, a Ag I AgBr reference electrode, and a 50-(im-thick fluorinated ethylene-propylene polymer membrane. Leland Clark, who invented the Clark oxygen electrode, also invented the glucose monitor and the heart-lung machine. 

Several types of reference electrodes are convenient for use in analytical electrochemistry. The use of high-input-impedance operational amplifiers in the reference electrode inputs of potentiostats ensures that very low levels of current are drawn from the reference electrode (see Chap. 6). This permits the use of reference electrodes that do not have to contain a large number of redox equivalents in order to ensure a constant reference potential and are therefore very small. Three reference-electrode designs that are convenient for use in analytical electrochemistry are shown in Figure 9.4. Saturated calomel and silver-silver chloride (of various concentrations of chloride) are among the most common commercially available or conveniently fabricated reference electrodes. 

The potential profile through the membrane that is placed between the sample and the internal reference solution was shown in Fig. 6.3. The composition of the internal solution can be optimized with respect to the membrane and the sample solution. In the interest of symmetry, it is advisable to use the same solvent inside the electrode as is in the sample. This solution also contains the analyte ion in the concentration, which is usually in the middle of the dynamic range of the response of the membrane. The ohmic contact with the internal reference electrode is provided by adding a salt that contains the appropriate ion that forms a fast reversible couple with the solid conductor. In recent designs, gel-forming polymers have been added into the internal compartment. They do not significantly alter the electrochemistry, but add mechanical stability and convenience of handling. 

Note that Equation 1.14 remains valid only if the chloride solution is saturated with chloride. This requires that this solution should be separated by the solution in which the compound(s) to be analysed is present. By using glass frits with small pore size that prevent the electrolyte solution from leaking out of the reference electrode, this problem can be solved, but because of the porous structure both solutions are electrolytically in contact, which is an important condition for performing electrochemistry. 

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