Ag electrodes

The standard states of Ag and of Ag (aq) have the conventional definitions, but there is an ambiguity in the definition of the standard state of e. Suppose that a reference electrode R is positioned above a solution of AgN03, which in turn is in contact with an Ag electrode. The Ag electrode and R are connected by a wire. Per Faraday, the processes are... 

Fig. V-17. Schematic diagram for the apparatus for measurement of Vobs (see text). The vibrating reference electrode is positioned close to the surface of a AgN03 solution in which there is an Ag electrode, which, in turn, is in electrical contact with the reference electrode. (From Ref. 196.)...

When first developed, potentiometry was restricted to redox equilibria at metallic electrodes, limiting its application to a few ions. In 1906, Cremer discovered that a potential difference exists between the two sides of a thin glass membrane when opposite sides of the membrane are in contact with solutions containing different concentrations of H3O+. This discovery led to the development of the glass pH electrode in 1909. Other types of membranes also yield useful potentials. Kolthoff and Sanders, for example, showed in 1937 that pellets made from AgCl could be used to determine the concentration of Ag+. Electrodes based on membrane potentials are called ion-selective electrodes, and their continued development has extended potentiometry to a diverse array of analytes. 

Fig. 1.20 Cell consisting of two reversible Ag /Ag electrodes (Ag in AgN03 solution). The rate and direction of charge transfer is indicated by the length and arrow-head as follows gain of electrons by Ag -he- Ag—> loss of electrons by Ag - Ag + e- —. (o) Both electrodes at equilibrium and (f>) electrodes polarised by an external source of e.m.f. the position of the electrodes in the vertical direction indicates the potential change. (K, high-impedance voltmeter A, ammeter R, variable resistance)...

The Ag/AgCl, Cl" electrode, which may be regarded as typical of electrodes of the second kind, consists of AgCl in contact with a soluble chloride, usually KCl. This electrode is essentially an Ag -F e Ag electrode, in which the 0 is controlled by the solubility product of AgCl and by the flci- Thus... 

On closing the external circuit between the two Ag electrodes, when a current flows, the net result will be simply to transfer an amount of solute from one solvent to the other, and the measured e.m.f. will be equal to the change in free energy associated with the transfer of a kome of ions from one solvent to the other. This quantity will contain, in addition to the usual communal term, a unitary term arising from the fact that, in the co-sphere of each positive ion and each negative ion, the amount of free energy lost by the dipoles of one solvent will be different from that lost by the dipoles of the other solvent. 

Frumkin was the first to give a qualitative consideration of the electrochemical properties of pc electrodes.10,20 70 He noted that the charge fixed value of the potential E and this may change the form of the capacitance curve near the diffuse-layer capacitance minimum. Important results were obtained in a pioneering paper by Valette and Hamelin.67 They compared experimental capacitance curves for a pc-Ag electrode and its three basic faces. They found that the capacitance of a pc-Ag electrode can be obtained by the superposition of the corresponding Cj, E curves for individual faces exposed at the pc surface, i.e. 

A weighted sum of C,E curves for the faces was found to be similar to the C,E curve for a pc electrode. According to Valette and Hamelin,67 all main Ag faces [(111), (100), and (110)] are exposed on the surface, their fractions 0j on the surface being 0.31, 0.23, and 0.46, respectively. These authors demonstrated that the diffuse-layer capacitance minimum potential E a of a pc-Ag electrode was only slightly less negative (30 mV) than the pzc of the Ag(110) face, i.e., for the face with the more negative value of EamQ. The diffuse-layer capacitance minimum for pc-Ag was wider and less deep than for the Ag faces. 

Leikis et al,223 used the Parsons-Zobel method to obtain the roughness factor fpz for pc/Ag electrodes. It was found that /pz 1.2, which was explained by the geometric inhomogeneity of the pc-Ag electrode surface. A more detailed analysis is given in Section II.2. Thus it should be noted that in the case of pc electrodes with appreciable differences of EamQvalues for the various planes (AEff o > 100 mV), it is impossible to obtain the true roughness coefficient, the actual Ea=0, and the inner-layer capacity. 

The electrical double-layer structure and fractal geometry of a pc-Ag electrode have been tested by Se vasty an ov et al.272 They found that the geometrical roughness of electrochemically polished pc-Ag electrodes is not very high (/pz 1.5 to 1.25), but the dependence of Chtr curves on cej, as well as on/pz, is remarkable (C, =30 to 80 fi cm-2 if/pz =1.5 to 1.0). 

The electrical double layer at an Ag electrode chemically polished in H20 + NaCN24 was studied using impedance and cyclic voltammetry.375... 

According to Vitanov et a/.,61,151 C,- varies in the order Ag(100) < Ag(lll), i.e., in the reverse order with respect to that of Valette and Hamelin.24 63 67 150 383-390 The order of electrolytically grown planes clashes with the results of quantum-chemical calculations,436 439 as well as with the results of the jellium/hard sphere model for the metal/electro-lyte interface.428 429 435 A comparison of C, values for quasi-perfect Ag planes with the data of real Ag planes shows that for quasi-perfect Ag planes, the values of Cf 0 are remarkably higher than those for real Ag planes. A definite difference between real and quasi-perfect Ag electrodes may be the higher number of defects expected for a real Ag crystal. 15 32 i25 401407 10-416-422 since the defects seem to be the sites of stronger adsorption, one would expect that quasi-perfect surfaces would have a smaller surface activity toward H20 molecules and so lower Cf"0 values. The influence of the surface defects on H20 adsorption at Ag from a gas phase has been demonstrated by Klaua and Madey.445... 

The temperature dependence of the electrical double-layer parameters has been determined for real393,398 as well as quasi-perfect Ag planes.382,394 For quasi-perfect Ag electrodes, the value of 3 ffa0/9rhas been found to be higher for Ag(100) than for Ag(lll), and so it was concluded that Ag(lll) is more hydrophilic than Ag(100). For real surfaces,382,385,386 dEff=0/BT increases in the order (110) < (100) <(111). The same order of planes has been observed for Au 446-448 BEa /BT linearly increases as AX (interfacial parameter) decreases, i.e., as the hydrophilicity of Ag and Au electrodes decreases.15 32 393 397 398 446 48... 

Wagner was first to propose the use of solid electrolytes to measure in situ the thermodynamic activity of oxygen on metal catalysts.17 This led to the technique of solid electrolyte potentiometry.18 Huggins, Mason and Giir were the first to use solid electrolyte cells to carry out electrocatalytic reactions such as NO decomposition.19,20 The use of solid electrolyte cells for chemical cogeneration , that is, for the simultaneous production of electrical power and industrial chemicals, was first demonstrated in 1980.21 The first non-Faradaic enhancement in heterogeneous catalysis was reported in 1981 for the case of ethylene epoxidation on Ag electrodes,2 3 but it was only... 

Similar is the behaviour with Ag electrodes deposited on YSZ as shown in Figures 5.40 and 5.41.68 The oxygen ion backspillover mechanism of electrochemical promotion is confirmed quite conclusively. 

Significant observations regarding the origin of NEMCA have been also made using Ultra-violet Photoelectron Spectroscopy (UPS) with Pt and Ag electrodes deposited on YSZ. In this case the work function of the electrode can be determined from the cutoff energy of secondary electrons (Fig. 5.43).24,68 As shown in Fig. 5.8b the change in the work function of the gas-exposed Ag surface is very close to the imposed electrode potential change AUwr. 

For the case of Ag it was found that both anodic and cathodic polarization lead to the creation of small insulated particles on the YSZ surface. Both the Fermi level and the work function of the insulated Ag particles was found to change with Ag electrode overpotential but the changes are smaller than on the continuous Ag film.24... 

T. Arakawa, A. Saito, and J. Shiokawa, Surface study of a Ag electrode on a solid electrolyte used as oxygen sensor, Applications of Surface Science 16, 365-372 (1983). 

The transition from case 1 (no ion spillover) to case 2 (ion spillover) is shown in Figure 7.11 for Pt and Ag electrodes deposited on YSZ. At low temperatures (no spillover) significant deviations from Equation (7.11) are observed. As temperature is increased these deviations vanish and Equation (7.11) is satisfied. 

Similar studies utilizing Au electrodes on YSZ showed again that the selectivity and yield of C2 hydrocarbons can be significantly affected by applying currents or potentials to the cell.40,41,53 The behaviour with Au appears to be qualitatively similar with that obtained with Ag electrodes although electrophilic behaviour is also reported.40,41... 

An important application of the Nernst equation is the measurement of concentration. In a concentration cell, the two electrodes are identical except for their concentrations. For such a cell, E° = 0 and at 25°C the potential corresponding to the cell reaction is related to Q by E = —(0.025693 V//z) In Q. For example, a concentration cell having two Ag+/Ag electrodes is... 

The cell shown in Figure 19-16 can serve as an example for calculations using Equation. One cell contains aqueous 1.00 M iron(in) chloride in contact with an iron metal electrode, and the other cell contains 1.00 M KCl in contact with a silver-silver chloride (AgCl/Ag) electrode. The half-reactions for these electrodes follow ... 

Pettinger, B., Philpott, M. R. and Gordon, J. G. (1981) Contribution of specifically adsorbed ions, water, and impurities to the surface enhanced Raman spectroscopy (SERS) of Ag electrodes. 

Historically, indicator electrodes have been metals which form a redox couple with the analyte, such as a Ag electrode for the determination of Ag", or a chemically inert metal which responds to the activity ratio of a soluble redox couple, such as a Pt electrode for Fe /Fe. Whereas simple indicator electrodes of this type perform well for the analysis of relatively pure samples, they are often subjwt to interferen< when apphed to complex samples such as those of biological origin. 

Metal insoluble-salt These consist of a metal in contact with one of its slightly soluble salts this salt in turn is in contact with a solution containing the anion of the salt. An example is represented as Ag AgCl Or (c). The electrode process at such an electrode as AgCl (s) Ag + Cl" Ag + e- —> Ag (s) or overall, AgCl (s) + e- Ag (s) + Cl". The electrode reaction involves only the concentration of Cl" as a variable, in contrast with the Ag Ag electrode, which has the Ag concentration as a variable. The most frequently electrode of this type is the calomel electrode (see text for description). 

The potential of a half-reaction is a measure of the disposition of that half-reaction to take place, no matter what the other half of the complete reaction is. Thus, the potential of any complete reaction can be obtained by adding potentials of its two half-reactions. The potential so obtained is a measure of disposition of the complete reaction to occur, and provides the voltage measured for a galvanic cell which was the overall reaction. For example, the entries in Table 6.11 for Ni and Ag electrodes are ... 

Although not essential, one often uses a previously coated AgCl-Ag electrode or a silver-plated Pt wire coated electrolytically in KC1 solution with a thin deposit of AgCl. Such dry AgCl-Ag electrodes are much in favour as reference electrodes (although in the absence of oxidants), in addition to calomel electrodes (Pt wire in contact with Hg, covered with calomel paste in contact with KC1 solution), which also belong to the second kind, viz.,... 

Their potentials in 0.1 N, lmolal, IN and saturated KC1 solutions are 0.3337, 0.2800, 0.2897 and 0.2415 V, respectively. The dilute types reach their equilibrium potentials more quickly and these potentials are less dependent on temperature the SCE has the advantage of being less sensitive to current flow (electrolysis). The AgCl-Ag electrodes are more compact, do not need a liquid function, which makes them exceedingly attractive for analysis in non-aqueous media, and support high temperatures. 

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