Silver standard electrode potentials

As may be seen from the diagram, silver in highly alkaline solution corrodes only within a narrow region of potential, provided complexants are absent. It is widely employed to handle aqueous solutions of sodium or potassium hydroxides at all concentrations it is also unaffected by fused alkalis, but is rapidly attacked by fused peroxides, which are powerful oxidising agents and result in the formation of the AgO ion Table 6.6 gives the standard electrode potentials of silver systems. 

Table 6.6 Standard electrode potentials of some silver systems ...

When metals are arranged in the order of their standard electrode potentials, the so-called electrochemical series of the metals is obtained. The greater the negative value of the potential, the greater is the tendency of the metal to pass into the ionic state. A metal will normally displace any other metal below it in the series from solutions of its salts. Thus magnesium, aluminium, zinc, or iron will displace copper from solutions of its salts lead will displace copper, mercury, or silver copper will displace silver. 

Worked Example 7.19 We immerse a piece of silver metal into a solution of silver ions at unit activity and at s.t.p. The potential across the cell is 0.799 V when the SHE is the negative pole. What is the standard electrode potential E of the Ag+, Ag couple ... 

To learn how to construct your own silver-silver chloride reference electrode, and learn how to test its quality with a simple test of its standard electrode potential C fgci Ag-... 

Both the ions of Ag+ and Cif are easily complexed by ammonia (amine) and the corresponding complexes are very stable [204]. In the system of silver-ammonia complex ions the oxidation-reduction standard electrode potential of silver is expressed by... 

Standard emf Values for the Cell H2/HCl/AgCl, Ag in Various Aqueous Solutions of Organic Solvents at Various Temperatures Temperature Dependence of the Standard Potential of the Silver Chloride Electrode Standard Electrode Potentials of Electrodes of the First Kind Standard Electrode Potentials of Electrodes of the Second Kind Polarographic Half-Wave Potentials (E1/2) of Inorganic Cations Polarographic E1/2 Ranges (in V vs. SCE) for the Reduction of Benzene Derivatives Vapor Pressure of Mercury... 

The following table lists the standard electrode potentials (in V) of some electrodes of the second kind.13 These consist of three phases. The metal is covered by a layer of its sparingly soluble salt and is immersed in a solution of a soluble salt of the anion. Equilibrium is established between the metal atoms and the solution anions through two partial equilibria one between the metal and its cation in the sparingly soluble salt and the other between the anion in the solid phase of the sparingly soluble salt and the anion in solution. The silver chloride electrode is preferred for precise measurements. 

Plot versus c and extrapolate to zero concentration to obtain (which is equal to the standard electrode potential for the silver-silver chloride electrode). If you wish, also calculate from Eq. (17), plot it versus /c, and extrapolate to obtain another value of as a check. 

In order to satisfy the necessary criteria, a reversible redox couple is utilized in the reference electrode half-cell reaction. The potential of a reversible reference electrode is thermodynamically defined by its standard electrode potential, EP (see for example Compton and Sanders, 1996, for further discussion). Currently, the most commonly used reference electrode in voltammetric studies is the silver/silver chloride electrode (3), which has overtaken the calomel electrode (see for example Bott, 1995) for which the reaction is (4). 

All cell potentials reached equilibrium in 1 or 2 hr, except when the solvent was anhydrous terf-butanol, in which the electrodes reached equilibrium only in dilute soltuions of HBr and even then only in a sluggish manner. This sluggish behavior has been reported (27) for the silver-silver bromide electrode in anhydrous ethanol when the acid was concentrated. In the dilute hydrobromic acid solutions used here, this phenomena was not observed in anhydrous ethanol. It is estimated that the standard electrode potential of the silver-silver bromide electrode in anhydrous terf-butanol is accurate to only d=l mV. However, these data are reported to the same degree of precision found in the other tert-buta-nol-water solvents in order to facilitate comparisons of the emf s in the various dilutions of tert-butanol used. 

The silver/silver chloride electrode. This is a silver wire coated with AgCl and immersed in a solution of constant cliloride concentration. The halfreaction AgCl -f e Ag -F Cr gives a stable, standard electrode potential of +0.20 V. 

The electrochemical series (Table 8.3) gives thermodynamic information on the so-called nobility of various metals the higher the standard electrode potential, the more noble is the metal, silver being more noble than Cu and Cu being more noble than Zn. 

Figure 18-7 Measurement of the electrode potential for an Ag electrode. If the silver ion activity in the right-hand compartment is 1.00, the cell potential is the standard electrode potential of the Ag /Ag half-reaction.

Because the silver electrode is on the right, the measured potential is, by definition, the standard electrode potential for the silver half-reaction, or the silver couple. Note that the silver electrode is positive with respect to the standard hydrogen electrode. Therefore, the standard electrode potential is given a positive sign, and we write... 

In contrast to the silver electrode, the cadmium electrode is negative with respect to the standard hydrogen electrode. Consequently, the standard electrode potential of the Cd/Cd " couple is by convention given a negative sign, and Ecd icd — —0.403 V. Because the cell potential is negative, the spontaneous cell reaction is not the reaction as written (that is, oxidation on the left and reduction on the right). Rather, the spontaneous reaction is in the opposite direction. 

If we proceed in the same way, we can obtain an expression for the standard electrode potential for the reduction of the thiosulfate complex of silver ion depicted in the third equilibrium shown at the start of this section. Here the standard potential is given by... 

DAS/SAH] Das, R. C., Sahu, G., Satyanarayana, D., Misra, S. N., Silver-silver selenocyanate electrode determination of standard electrode potential, solubility product of AgSeCN and various thermodynamic parameters at 35, 40, 45 and 50°C, Electrochim. Acta, 19, (1974), 887-890. Cited on page 307. 

The standard electrode potentials for (Zn /Zn) and (Ag /Ag) are, from Table 19-1, -0.763 V and -1-0.799 V, respectively. The standard potential of the cell is the difference between these two numbers, 0.799-(-0.763) = 1.562 V. The silver electrode potential is higher, and thus silver ion is the oxidizing agent. The zinc couple provides the reducing agent and is the negative electrode. The equation is... 

From an appropriate plot of these data calculate °, the standard electrode potential of the silver-silver chloride electrode in methanol solvent. 

The standard electrode potential for the silver-silver chloride electrode is thus 0.2224 V. 

Thus, when in contact with a solution saturated with silver iodide, the potential of a silver electrode can be described either in terms of the silver ion activity (with the standard electrode potential for the simple silver half-reaction) or in terms of the iodide ion activity (with the standard electrode potential for the silver-silver iodide half-reaction). The silver -silver iodide half-reaction is usually more convenient. 

Data for the potential of the silver electrode in the presence of selected ions are given in the tables of standard electrode potentials in Appendix, 5 and in Table 22-1. Similar information is also provided for other electrode systems. These data often simplify the calculation of half-cell potentials. 

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