Ag/AgCl

Several designs for STM electrochemical cells have appeared in the literature [M]- hr addition to an airtight liquid cell and the tip insulation mentioned above, other desirable features include the incorporation of a reference electrode (e.g. Ag/AgCl in saturated KCl) and a bipotentiostat arrangement, which allows the independent control of the two working electrodes (i.e. tip and substrate) [ ] (figure BL19.11). 

The Ag (100) surface is of special scientific interest, since it reveals an order-disorder phase transition which is predicted to be second order, similar to tire two dimensional Ising model in magnetism [37]. In fact, tire steep intensity increase observed for potentials positive to - 0.76 V against Ag/AgCl for tire (1,0) reflection, which is forbidden by symmetry for tire clean Ag(lOO) surface, can be associated witli tire development of an ordered (V2 x V2)R45°-Br lattice, where tire bromine is located in tire fourfold hollow sites of tire underlying fee (100) surface tills stmcture is depicted in tlie lower right inset in figure C2.10.1 [15]. 

Ag/AgCl, satd. KCl Ag/AgCl, OlMKCl Hg/HgO, l.OMNaOH Hg/HgO, O.lMNaOH Hg/Hg2S04, satd. K2SO4 (22°C) Hg/Hg2S04, satd. KCl... 

Solvent, wt % Methanol, Ag/AgCl Ethanol, Ag/AgCl 2-Propanol, Ag/AgCl Acetone, Ag/AgCl Dioxane, Ag/AgCl Ethylene glycol, Ag/AgCl Methanol, calomel Dioxane, calomel... 

As with the saturated calomel electrode, the potential of the Ag/AgCl electrode is determined by the concentration of Cb used in its preparation. 

When prepared using a saturated solution of KCl, the Ag/AgCl electrode has a potential of +0.197 V at 25 °C. Another common Ag/AgCl electrode uses a solution of 3.5 M KCl and has a potential of +0.205 at 25 °C. The Ag/AgCl electrode prepared with saturated KCl, of course, is more temperature-sensitive than one prepared with an unsaturated solution of KCl. 

A typical Ag/AgCl electrode is shown in figure 11.9 and consists of a silver wire, the end of which is coated with a thin film of AgCl. The wire is immersed in a solution that contains the desired concentration of KCl and that is saturated with AgCl. A porous plug serves as the salt bridge. The shorthand notation for the cell is... 

One example of a liquid-based ion-selective electrode is that for Ca +, which uses a porous plastic membrane saturated with di-(n-decyl) phosphate (Figure 11.13). As shown in Figure 11.14, the membrane is placed at the end of a nonconducting cylindrical tube and is in contact with two reservoirs. The outer reservoir contains di-(n-decyl) phosphate in di- -octylphenylphosphonate, which soaks into the porous membrane. The inner reservoir contains a standard aqueous solution of Ca + and a Ag/AgCl reference electrode. Calcium ion-selective electrodes are also available in which the di-(n-decyl) phosphate is immobilized in a polyvinyl chloride... 

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). 

A novel sensitive and seleetive adsorptive stripping proeedure for simultaneous determination of eopper, bismuth and lead is presented. The method is based on the adsorptive aeeumulation of thymolphetalexone (TPN) eomplexes of these elements onto a hanging mereury drop eleetrode, followed by reduetion of adsorbed speeies by voltammetrie sean using differential pulse modulation. The optimum analytieal eonditions were found to be TPN eoneentration of 4.0 p.M, pH of 9.0, and aeeumulation potential at -800 mV vs. Ag/AgCl with an aeeumulation time of 80 seeonds. The peak eurrents ai e proportional to the eoneentration of eopper, bismuth and lead over the 0.4-300, 1-200 and 1-100 ng mL ranges with deteetion limits of 0.4, 0.8 and 0.7 ng mL respeetively. The proeedure was applied to the simultaneous determination of eopper, bismuth and lead in real and synthetie samples with satisfaetory results. 

The effeet of eleetroehemieal pretreatment was evaluated. We observed that eleetroehemieal anodization at 1.9 V vs Ag/AgCl for 2 min and reduetion step at -0. IV vs Ag/AgCl for 30 see ean improve the rate of eharge transfer about one order of magnitude for hexaeyanoferrate redox eouple and also ehange surfaee mierostrueture. We used Raman speetroseopy to probe these ehanges. 

In order to find optimal conditions for the soluble copper determination we examined the influence of electrolysis potential, electrolysis time, and the solution stirring rate on the accuracy and sensitivity of determination. We found that the optimal parameters for PSA determination of copper were electrolysis potential of -0.9 V vs. 3.5 mol/dm Ag/AgCl, electrolysis time of 300 s, and solution stirring rate of 4000 rpm. The soluble copper content in samples investigated in this study varied from 1.85 to 4.85 ppm. Very good correlation between the copper content determined by PSA and AAS indicated that PSA could be successfully applied for the soluble copper content determination in various dental materials. 

The reference potential of the Ag-AgCl electrode in brackish water must be eorreeted for ehloride ion eontent (i.e., a ehange in ehloride ion eoneentration by a faetor of 10 shifts the referenee potential by about 50 mV in the positive direetion... 

Marine structure Reference electrode Cu-CuS04 (V) Ag-AgCl in seawater (V) Uzn (V)... 

Ag-AgCl electrodes are usually chosen (see Section 16.7 and Table 3-1). Care has to be taken that the electrical connection to the ship is sufficiently low resistance... 

Since cathodic protection of concrete structures in the United States has been very much advanced, protection criteria have been developed [46]. They correspond to the pragmatic criteria Nos. 3 and 4 in Table 3-3 (see Section 3.3.3.1). It is assumed that the protective effect is adequate if, upon switching off the protection current, the potential becomes more than 0.1 V more positive within 4 hours. The measurements are carried out in various parts of the protected object with built-in Ag-AgCl reference electrodes or with any electrodes on the external surface. 

As an example. Fig. 20-7 shows potential and protection currents of two parallel-connected 750-liter tanks as a function of service life. The protection equipment consists of a potential-controlled protection current rectifier, a 0.4-m long impressed current anode built into the manhole cover, and an Ag-AgCl electrode built into the same manhole [10,11]. A second reference electrode serves to control the tank potential this is attached separately to the opposite wall of the tank. During the whole of the control period, cathodic protection is ensured on the basis of the potential measurement. The sharp decrease in protection current in the first few months is due to the formation of calcareous deposits. 

Potential control with zinc reference electrodes presented a problem because deposits of corrosion products are formed on zinc in hot water. This caused changes in the potential of the electrode which could not be tolerated. Other reference electrodes (e.g., calomel and Ag-AgCl reference electrodes) were not yet available for this application. Since then, Ag-AgCl electrodes have been developed which successfully operate at temperatures up to 100°C. The solution in the previous case was the imposition of a fixed current level after reaching stationary operating conditions [27]. 

The impressed current method with metal oxide-coated niobium anodes is usually employed for internal protection (see Section 7.2.3). In smaller tanks, galvanic anodes of zinc can also be used. Potential control should be provided to avoid unacceptably negative potentials. Pure zinc electrodes serve as monitoring and control electrodes in exposed areas which have to be anodically cleaned in the course of operation. Ag-AgCl electrodes are used to check these reference electrodes. 

A reference electrode is fixed in each protection region in the most unfavorable place for current distribution and serves to regulate the potential. The Ag-AgCl reference electrodes in the turbine sections (El to E4) have a threaded connection that will withstand up to 60 bars of pressure. The electrode E5 in the box header is pure zinc. 

Half-cells based on Ag-AgCl, Hg-HgO, Hg-HgSQj and other system.s can be used as reference electrodes (see Table 3-1) 14]. Electrodes have been developed to operate up to 100 bar and 250°C (see Fig. 21-8). 

Alloy Anode potential (V vs. Ag/AgCl/Seawatcr) Max current capacity (Ah/kg)... 

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